The Bottom Line

Subungual melanoma is a form of melanoma that develops under the nail — most often the thumbnail or big toenail. It accounts for only 0.7-3.5% of melanomas in white populations but 15-35% of melanomas in people with darker skin tones. It usually appears as a dark brown or black vertical streak in the nail, and it is frequently diagnosed late because it is mistaken for a bruise or a normal pigmented band. Early diagnosis is life-saving — if you have a new, growing, or unusual dark streak under a nail, see a dermatologist promptly.

What Is Subungual Melanoma?

Melanoma is a cancer that starts in the melanocytes — the cells that produce pigment. While most melanomas arise in sun-exposed skin, subungual melanoma (also called nail unit melanoma) develops in the nail matrix, the tissue under the base of the nail that generates new nail cells. As cancerous cells produce abnormal pigment, a dark streak grows from the base of the nail toward the free edge — this is called melanonychia striata.

Subungual melanoma is part of a broader subtype called acral lentiginous melanoma (ALM), which occurs on the palms, soles, and nail units. Unlike most other melanomas, ALM is not caused by sun exposure, which explains why it occurs at similar rates in sun-protected people of color as it does in fair-skinned people.

It most commonly affects the thumb and big toe (the nails with the widest matrix). It occurs most often in adults over 50 years old, though it can develop at any age.

What Does It Look Like? Recognizing the Warning Signs

Not every dark streak under a nail is melanoma — nail matrix nevi (moles) and other benign causes can produce similar-looking bands. However, certain features are more worrying and require evaluation:

The ABCDE of Nail Melanoma

Dermatologists apply a modified version of the melanoma ABCDE criteria specifically to nail pigmentation:

  • A - Age and race: New dark streaks in adults over 50, or in people with darker skin (African, Asian, Hispanic descent) warrant closer attention
  • B - Band characteristics: A band wider than 3 mm, brown to black, with blurred or irregular borders is more suspicious than a narrow, uniform, well-defined band
  • C - Change: Any rapid change in color, width, or band characteristics is a red flag
  • D - Digit involved: The thumb, index finger, and big toe are the most common sites for subungual melanoma
  • E - Extension: Hutchinson's sign — dark pigmentation spreading from the nail onto the skin of the cuticle, the nail fold, or the fingertip — is a highly significant warning sign

Hutchinson's Sign

This finding deserves special emphasis. When melanoma cells spread beyond the nail matrix, they deposit pigment into the surrounding skin, creating a dark shadow on the proximal nail fold (the cuticle skin), the lateral nail folds, or the fingertip skin. Any new dark discoloration of the skin around a pigmented nail streak should be evaluated urgently.

Advanced Presentation

Because subungual melanoma is so often initially mistaken for a bruise or fungal infection, many cases are not diagnosed until the tumor has grown significantly. Advanced subungual melanoma may appear as:

  • Destruction or distortion of the nail plate
  • A raised mass or nodule under or at the nail edge
  • Bleeding under or around the nail without obvious trauma
  • An ulcerated or non-healing wound around the nail

Why Is It Often Diagnosed Late?

Several factors contribute to delayed diagnosis:

  • Mistaken for a bruise: Many patients assume a dark streak is from an old injury and wait for it to grow out — which it does not do if it is melanoma, because the cancerous matrix keeps producing pigment
  • Mistaken for fungal infection: Nail dystrophy accompanying subungual melanoma is often attributed to onychomycosis, and months of antifungal treatment may be tried before a biopsy is considered
  • Reassurance from previous normal streaks: People with naturally darker nails may have benign pigmented bands from childhood and assume a new change is the same thing
  • Hidden location: The nail covers the tumor, making it easy to overlook

The average time from the first appearance of symptoms to diagnosis is reported at 1.5-2 years in multiple studies — a significant delay that worsens prognosis.

Diagnosis

Dermoscopy

Your dermatologist will use a handheld magnifying tool called a dermoscope to examine the nail streak in detail. Features such as irregular band color, irregular spacing of pigment lines, and Hutchinson's sign can be assessed more precisely with dermoscopy than with the naked eye.

Nail Biopsy

A nail matrix biopsy is the definitive way to diagnose or rule out subungual melanoma. The procedure involves anesthesia of the finger (digital nerve block), temporary removal of part or all of the overlying nail plate, and excision of a small piece of the nail matrix tissue from the area producing the pigment. This is then examined by a dermatopathologist. Delaying biopsy out of fear of the procedure is one of the leading causes of late-stage diagnosis.

Treatment

Surgery

Surgery is the primary treatment. The approach depends on tumor thickness (Breslow depth) and staging:

  • Wide local excision: For early, thin melanomas (Breslow depth less than 1 mm), excision of the nail unit with adequate margins may preserve the finger. Current evidence supports more conservative approaches for thin tumors
  • Amputation (digital or ray amputation): Still recommended for thicker tumors or when margins cannot otherwise be achieved
  • Sentinel lymph node biopsy: Recommended for tumors greater than 1 mm thick to assess whether cancer cells have spread to nearby lymph nodes

Advanced Disease

When melanoma has spread to lymph nodes or distant organs, systemic treatments are used:

  • Immunotherapy: Checkpoint inhibitors (nivolumab, pembrolizumab, ipilimumab) activate the immune system to fight the cancer
  • Targeted therapy: For tumors with BRAF V600E mutation, BRAF/MEK inhibitors (vemurafenib, dabrafenib + trametinib) can be used — though this mutation is less common in acral lentiginous melanoma than in other subtypes
  • Radiation therapy: Sometimes used for symptom management or for brain metastases

Prognosis

Five-year survival rates for subungual melanoma are significantly lower than for melanomas of similar thickness on other body sites, primarily because of the delay in diagnosis. When caught at stage I (thin, localized), 5-year survival exceeds 80%. This underscores why early evaluation of suspicious nail streaks is so critical.

When to See a Dermatologist

  • You have a new dark brown or black streak running the length of a nail that was not there before
  • A pigmented nail streak is getting wider, darker, or more irregular
  • You notice dark discoloration of the skin around a pigmented nail (Hutchinson's sign)
  • You are of African, Asian, or Hispanic descent and develop any new nail pigmentation in adulthood
  • You have been treating a suspected nail fungus for months without improvement
  • You have a nail that is breaking down, bleeding, or ulcerating without a clear cause

Frequently Asked Questions

Is every dark streak in a nail melanoma?

No — most are not. Benign causes of nail pigment streaks include nail matrix nevi (moles), racial melanonychia (especially common in people with darker skin tones), trauma, systemic diseases, and medications (including chemotherapy, antimalarials, and minocycline). However, only a dermatologist can clinically assess a streak, and some cases will require a biopsy to be certain. It is always better to have the nail evaluated than to assume it is benign.

I have multiple dark streaks — is that more or less worrying?

Multiple light-colored or gray streaks across many nails are usually benign, particularly in people with darker skin tones (racial melanonychia). Subungual melanoma is typically a single, prominent, dark, and growing streak in one nail. Multiple streaks can also result from medications or systemic diseases. A dermatologist can assess whether the pattern is reassuring or not.

My nail turned dark after an injury — could it be melanoma?

A uniform dark color across the nail following an acute injury is almost always a subungual hematoma — pooled blood under the nail. It typically grows out with the nail over weeks to months. If the dark color is not clearly associated with an injury, appears as a streak rather than a blotchy pooling, or does not grow out and disappear, it should be evaluated. When in doubt, see a dermatologist.

What happens if I need a nail biopsy?

A nail matrix biopsy is performed with a digital nerve block (local anesthesia injected into the base of the finger) so you should feel no pain during the procedure. The overlying nail plate is gently separated, and a small piece of matrix tissue is removed. After the procedure, the finger is bandaged. Some temporary nail distortion is possible, but permanent nail changes from a small biopsy are uncommon. The procedure is far preferable to missing a melanoma at an early, curable stage.

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Trusted Resources

Always consult a board-certified dermatologist for an accurate diagnosis and personalized treatment plan. This article is for educational purposes only and does not replace professional medical advice.