Melanonychia

What is it?

Melanonychia (also called longitudinal melanonychia or melanonychia striata) is a brown to black pigmented streak that runs lengthwise along the nail, from the base to the free edge. It is caused by melanin — the same pigment that gives skin and hair their color — being deposited into the nail plate as it grows.

Melanonychia is extremely common in individuals with darker skin tones, where it occurs naturally and affects multiple nails. In lighter-skinned individuals it is less frequent, and when it appears — particularly on a single nail in an adult — it requires careful evaluation to distinguish harmless causes from nail melanoma.

In most cases, melanonychia is benign. The pigmented band is produced by melanocytes (pigment-producing cells) in the nail matrix (the tissue at the base of the nail where the nail plate is formed). These cells may become more active without multiplying (melanocytic activation) or they may increase in number (melanocytic hyperplasia), as seen in nail moles (nevi), freckles (lentigines), and rarely, melanoma.

What does it look like?

• Appearance: A longitudinal (vertical) band of pigment running from the nail base to the tip
• Color: Ranges from light brown or gray to dark brown or black
• Width: Can be a narrow line (1–2 mm) or a broad band (>3 mm); very wide bands require medical evaluation
• Number: May involve one nail or multiple nails simultaneously
• Borders: Benign bands tend to have regular, well-defined edges; irregular or blurry borders are concerning
• Nail surface: The nail itself may be smooth and normal, or it may show ridging, splitting, or dystrophy in more concerning cases

Where does it appear?

• Fingernails: Most commonly the thumbnail, followed by the index finger
• Toenails: The great toenail is most frequently affected
• Single nail vs. multiple nails: A single pigmented band in a light-skinned adult warrants attention; multiple bands on several nails are common in individuals with darker skin and are usually physiologic
• Periungual skin: Pigment that extends beyond the nail onto the surrounding skin (Hutchinson sign) may indicate melanoma

Associated features

• In physiologic melanonychia: Multiple uniform, thin bands on several nails; no nail damage; common in darker skin types; increases with age
• In trauma-related melanonychia: Dark spots or streaks associated with nail biting, tight shoes, or repetitive pressure; may include tiny blood spots visible on dermoscopy
• In nail moles or freckles: Single-digit band, usually narrow (<5 mm), homogeneous color; the nail surface is intact
• In nail melanoma: A single, broad (>3 mm) band with irregular borders, color variation within the band, progressive widening, nail plate damage, and possible pigmentation of surrounding skin

When to see a doctor

Seek medical evaluation for:

• A new pigmented band on a single nail, especially if you are an adult with lighter skin
• A band that is wider than 3 mm or that is getting wider over time
• A pigmented band with uneven color (mixed light and dark areas) or irregular borders
• Pigment spreading from the nail onto the surrounding skin (cuticle, lateral nail folds)
• A pigmented nail that also shows cracking, splitting, or thickening
• Any change in a pre-existing pigmented band — darkening, widening, or blurring of edges
• A family history of melanoma combined with a new pigmented nail band