Q: I have dark vertical lines on some of my fingernails. Overall, I'm healthy. What causes this condition?

A: You probably have a harmless condition called melanonychia.

It features brown or black streaks of the nail. It occurs because of increased pigmentation.

Melanonychia is more common in older people and darker-skinned people. It can also be seen after radiation therapy, after ultraviolet light exposure, while taking certain medications, during pregnancy, and with certain medical conditions. Melanonychia is rare in children.

Moles in the nail can also appear as dark streaks.

It is very important to have a dark streak in a nail examined by a doctor because melanoma, a dangerous form of skin cancer, can also appear as a dark nail streak. This type of melanoma is called "subungual melanoma." It will sometimes show a dark pigmentation on the skin at the edge of the streaked nail. This is called "Hutchinson's sign" and can be helpful in diagnosis. In some cases, your doctor may want to do a biopsy.

Rebecca Campen, M.D., J.D., is an Assistant Professor of Dermatology at Harvard Medical School and former Deputy Director of the Harvard/Massachusetts General Hospital Cutaneous Biology Research Center (CBRC). She currently serves as senior advisor to the CBRC and divides her time between clinical practice of dermatology at the Massachusetts General Hospital and private practice in Savannah, Georgia. Dr. Campen is the author of two books for medical students and residents, Going Into Medical Practice and Blueprints in Dermatology.