Black Streaks in Toenails & Fingernails — Causes, Warning Signs & When to Worry

Quick answer:A dark vertical streak in a finger- or toenail is often harmless pigment, but a new, widening, or single dark band — especially with color spreading into the surrounding skin — can be a sign of melanoma and should be checked promptly. A podiatrist or dermatologist can evaluate and biopsy if needed. Call (810) 206-1402.

✅ Medically reviewed by Dr. Tom Biernacki, DPM — Board-Certified Podiatrist · Last updated April 7, 2026

What Causes Black Streaks in Toenails and Fingernails?

A dark streak in the nail can come from several sources, ranging from completely harmless to medically urgent. Here are the six most common causes, listed from most to least frequent:

1. Longitudinal Melanonychia (Benign Pigment Streak)

The most common cause of a dark streak in the nail. Melanocytes (pigment-producing cells) in the nail matrix become activated and produce melanin that gets incorporated into the nail plate as it grows. The result is a brown or black vertical line running from the cuticle to the tip. This is extremely common in people with darker skin tones — affecting up to 77% of African Americans and 10–20% of Japanese individuals over age 50. It is also seen in lighter-skinned individuals, though less frequently.

2. Subungual Hematoma (Bruise Under the Nail)

Blood trapped between the nail plate and nail bed from trauma — stubbing the toe, dropping something heavy, or repetitive impact from running. The blood appears as a dark red, purple, or black spot or streak. The key distinguishing feature: a hematoma moves distally (toward the tip) as the nail grows out and eventually disappears. It does not originate from the cuticle.

3. Fungal Nail Infection

Certain species of dermatophytes and molds can produce dark pigment in the nail plate. Fungal discoloration typically appears as dark brown, greenish-black, or streaky discoloration with accompanying nail thickening and crumbling. The discoloration pattern is usually irregular and involves the nail plate itself, not just a single clean line.

4. Medication-Induced Melanonychia

Several medications can trigger melanocyte activation in the nail matrix. The most common culprits include certain chemotherapy drugs (doxorubicin, cyclophosphamide), antimalarials (hydroxychloroquine), AZT (zidovudine), and some antibiotics. The streaks typically affect multiple nails and began appearing after starting the medication.

5. Subungual Melanoma

The most dangerous cause, though also the rarest. Subungual melanoma accounts for only about 0.7–3.5% of all melanomas, but it has a poorer prognosis than cutaneous melanoma because it’s often diagnosed late. It occurs most frequently in people aged 40–70 and disproportionately affects individuals with darker skin — melanoma of the nail unit accounts for up to 20% of all melanomas in African Americans compared to less than 2% in Caucasians.

6. Splinter Hemorrhages

Tiny longitudinal lines of blood caused by damage to small blood vessels in the nail bed. They appear as thin red or dark lines that run vertically along the nail. Most are caused by minor trauma, but multiple splinter hemorrhages in several nails can occasionally indicate endocarditis (heart valve infection) or vasculitis. Isolated splinter hemorrhages in one or two nails are almost always harmless.

Benign vs. Dangerous — The ABCDEF Rule

Dermatologists and podiatrists use a standardized screening tool called the ABCDEF rule to evaluate dark nail streaks. This is the same framework we use at Balance Foot & Ankle during clinical evaluation:

Letter	Stands For	Concerning Feature
A	Age / African American / Asian	Peak incidence 40–70 years; higher risk in darker skin tones
B	Band (brown/black)	Width ≥3mm, irregular borders, varied coloring within the band
C	Change	Band getting wider, darker, or changing shape over time
D	Digit	Thumb, big toe, or index finger most commonly affected
E	Extension	Hutchinson sign — pigment extending from the nail onto the cuticle, nail fold, or fingertip
F	Family/personal history	Personal or family history of melanoma; prior atypical moles

The most critical sign is E — the Hutchinson sign. When dark pigment extends from the nail bed onto the surrounding skin (the cuticle area or nail folds), this strongly suggests subungual melanoma. If you can see dark pigment on the skin around the nail — not just on the nail plate — seek evaluation immediately.

Longitudinal Melanonychia — When Dark Streaks Are Normal

Longitudinal melanonychia (LM) is the most common cause of dark nail streaks and is completely benign. Here’s what normal melanonychia looks like:

Typical features of benign melanonychia: A single, uniform brown or tan band running vertically from the cuticle to the nail tip. The band has well-defined, parallel borders. Width is typically 2–4mm. The color is even throughout (not splotchy or varied). It has been stable in size and color over months to years. Most common in adults with darker skin tones, but can appear in anyone.

Who gets it: Up to 77% of African Americans have at least one nail with longitudinal melanonychia by age 20. It’s also common in Hispanic, Asian, and Middle Eastern individuals. In lighter-skinned people, it’s less common but not unusual — especially after age 40 or following nail trauma that activates dormant melanocytes.

Does it need treatment? No. Benign melanonychia is a cosmetic finding that requires no treatment. However, we recommend annual monitoring — photograph the streak yearly and compare for any changes in width, color, or border regularity. Any evolution should prompt re-evaluation.

Subungual Hematoma — Blood Under the Nail

A subungual hematoma is a collection of blood under the nail plate, almost always caused by trauma. It’s one of the most common reasons patients come to us concerned about a “black spot” on their toenail.

How to distinguish from melanoma: A hematoma is a spot or stain, not a longitudinal band. It does not extend from the cuticle. Over weeks, it migrates toward the tip of the nail as the nail grows — track its position with a photo every 2 weeks. If the dark area is moving forward, it’s blood. If it stays anchored at the base, it may be pigment and needs evaluation.

Treatment: Most subungual hematomas resolve on their own as the nail grows out (3–6 months for fingernails, 6–12 months for toenails). If the hematoma is acute (less than 48 hours) and causing significant throbbing pain, a podiatrist can drain it by creating a small hole in the nail plate — this provides immediate pain relief. If the hematoma involves more than 50% of the nail bed or is accompanied by a nail fracture, the nail may need to be removed for proper evaluation of the underlying bed.

Fungal Nail Discoloration

Toenail fungus can produce dark pigmentation, particularly infections caused by molds (non-dermatophyte molds) or Trichophyton rubrum. The pattern is usually distinguishable from melanocytic streaks because fungal discoloration is accompanied by nail thickening, crumbling, and debris under the nail — features absent in melanonychia and melanoma.

If your dark streak is accompanied by a thick, crumbly nail texture, the cause is very likely fungal. A nail culture or clipping analysis can confirm. Treatment with oral antifungals (terbinafine 250mg daily for 12 weeks) resolves both the infection and the dark discoloration.

Subungual Melanoma — What You Need to Know

Subungual melanoma is rare, but it deserves direct discussion because early detection saves lives. Bob Marley died at age 36 from acral melanoma that started under his toenail — a case that underscores how seriously this diagnosis should be taken.

Key facts about subungual melanoma:

It accounts for less than 3.5% of all melanomas but carries a worse prognosis because of delayed diagnosis. The 5-year survival rate when caught early (confined to the nail unit) is approximately 80%. When diagnosed late (spread beyond the nail), survival drops to 15–30%. The big toe and thumb are the most commonly affected digits. It is NOT caused by sun exposure — the cause is unknown.

What it looks like: A longitudinal dark band (brown, black, or blue-black) that is wider than 3mm, has irregular or blurred borders, shows color variation within the band, and progressively widens over weeks to months. The most alarming sign is the Hutchinson sign — pigment extending from under the nail onto the cuticle or surrounding skin. Some subungual melanomas are amelanotic (non-pigmented), presenting as a persistent red or pink streak, which makes them even harder to catch early.

What we do: If the ABCDEF screening raises concern, we perform a nail matrix biopsy. This involves numbing the toe, reflecting the nail fold, and taking a small sample from the pigmented area of the nail matrix. The tissue goes to dermatopathology for microscopic analysis. Results typically return within 7–10 days. If melanoma is confirmed, referral to surgical oncology for definitive excision is immediate.

Warning Signs — When to See a Doctor Immediately

We want to be clear: most dark nail streaks are harmless. But because the consequence of missing a melanoma is so severe, we always recommend professional evaluation for any streak you’re uncertain about. A quick office visit and, if needed, a small biopsy can provide definitive peace of mind. At Balance Foot & Ankle, we perform nail biopsies in-office at both our Howell and Bloomfield Township locations.

How We Diagnose Black Nail Streaks

When you come in with a dark nail streak, here’s the evaluation process we follow:

Clinical examination: We assess the streak using the ABCDEF criteria, examine the nail with magnification, check for Hutchinson sign, and review your medical history. We ask about trauma, medications, family history of melanoma, and how long the streak has been present.

Dermoscopy: A dermatoscope (a specialized magnifying device with polarized light) allows us to see the pigment pattern within the nail plate at high magnification. Benign melanonychia shows regular, parallel lines of pigment. Melanoma shows irregular lines, loss of parallelism, and color variation. Dermoscopy improves diagnostic accuracy significantly compared to naked-eye examination alone.

Nail matrix biopsy: If clinical examination and dermoscopy raise concern, we perform a biopsy of the nail matrix — the tissue at the base of the nail where the pigment originates. This is done under local anesthesia in our office. The procedure takes about 20 minutes, and most patients return to normal activity the next day. Biopsy provides a definitive pathologic diagnosis.

Nail clipping analysis: For cases where the clinical picture suggests fungal discoloration rather than melanocytic origin, we can send a nail clipping for fungal culture and microscopy. This is non-invasive and helps rule out fungal causes before considering biopsy.

Treatment Options by Cause

Cause	Treatment	Timeline
Benign melanonychia	No treatment needed; annual photo monitoring	Ongoing
Subungual hematoma	Observation or trephination (drainage) if painful	Resolves in 3–12 months as nail grows out
Fungal discoloration	Oral terbinafine 250mg × 12 weeks; topical antifungals	6–9 months for full nail regrowth
Medication-induced	Discuss with prescribing physician; may resolve if medication is changed	Months after discontinuation
Subungual melanoma	Surgical excision (wide local excision or amputation depending on stage)	Immediate referral to oncology
Splinter hemorrhages	None if isolated; medical workup if multiple nails affected	Resolves with nail growth

In-Office Treatment at Balance Foot & Ankle

If home treatment isn’t providing relief for your foot and ankle conditions, our podiatry team at Balance Foot & Ankle can help with same-day evaluations and advanced in-office care.

Frequently Asked Questions

Are black lines in nails always cancer?

No — the vast majority of dark lines in nails are benign. Longitudinal melanonychia (normal pigment activation) is by far the most common cause, especially in people with darker skin tones. Subungual melanoma is rare, accounting for less than 3.5% of all melanomas. However, because any dark streak could be melanoma, professional evaluation is recommended for new or changing streaks.

What does melanoma look like on a toenail?

Subungual melanoma typically appears as a dark longitudinal band wider than 3mm with irregular borders, variable coloring (multiple shades of brown and black within the same band), and progressive widening over time. The most specific sign is the Hutchinson sign — dark pigment extending from the nail onto the surrounding cuticle or skin. The big toe is the most commonly affected.

Should I see a podiatrist or dermatologist for a dark nail streak?

Either can evaluate nail streaks effectively. Podiatrists are specialists in toenail conditions and perform nail biopsies routinely. Dermatologists have expertise in pigmented lesions and dermoscopy. For toenail streaks, a podiatrist is often the most convenient first step — we can perform the evaluation and biopsy in one visit, or refer to dermatology-oncology if needed.

Can toenail fungus cause a black streak?

Yes. Certain fungal species, particularly molds and some dermatophytes, can produce dark brown or black pigmentation in the nail plate. Fungal discoloration is usually accompanied by nail thickening, crumbling, and debris — features that help distinguish it from melanocytic causes. A nail clipping sent for fungal culture provides definitive diagnosis.

The Bottom Line

A dark streak in your toenail or fingernail is usually benign — most often longitudinal melanonychia, a subungual hematoma, or fungal discoloration. But because subungual melanoma can present identically in its early stages and carries serious consequences when missed, we recommend professional evaluation for any new, changing, or wide (>3mm) dark streak. A simple office visit with possible dermoscopy can usually determine whether you need a biopsy or just monitoring. When in doubt, get it checked — early detection of subungual melanoma dramatically improves outcomes.

Sources

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3. Tan KB, Moncrieff M, Thompson JF, et al. “Subungual melanoma: a study of 124 cases.” Australas J Dermatol. 2007;48(1):1-8.
4. Haneke E, Baran R. “Longitudinal melanonychia.” Dermatol Surg. 2001;27(6):580-584.