Background: Subungual melanoma is a relatively rare disease with reported incidence between 0.7% to 3.5% of all melanoma cases in the general population. Unlike the significant improvement in the diagnosis of cutaneous melanoma, the diagnosis of subungual melanoma has shown little, if any, improvement over the years. The widespread adoption of the ABCDs of cutaneous melanoma has helped increase public and physician awareness, and thus helped increase the early detection of cutaneous melanoma; the same criteria cannot be applied to the examination of the nail pigmentation. Objective: We reviewed the world literature on subungual melanoma and arranged the available information into a system for the identification of subungual melanoma. This system has to be thorough, easy to remember, and easy to apply by both physician and lay public. A case to illustrate the delayed diagnosis often encountered in the current evaluation of nail melanoma is presented. Methods: A thorough review of the world literature on subungual melanoma was undertaken. The important findings of various studies and case reports were compared among themselves and the salient features were summarized. The information was then categorized under the easily recalled letters of the alphabet, ABCD, that have already become associated with melanoma. Results: The most salient features of subungual melanoma can be summarized according to the newly devised criteria that may be categorized under the first letters of the alphabet, namely ABCDEF of subungual melanoma. In this system A stands for a ge (peak incidence being in the 5th to 7th decades of life and A frican Americans, A sians, and native A mericans in whom subungual melanoma accounts for up to one third of all melanoma cases. B stands for brown to black b and with breadth of 3 mm or more and variegated borders. C stands for c hange in the nail band or lack of change in the nail morphology despite, presumably, adequate treatment. D stands for the d igit most commonly involved; E stands for e xtension of the pigment onto the proximal and/or lateral nailfold (ie, Hutchinson’s sign); and F stands for f amily or personal history of dysplastic nevus or melanoma. Conclusion: Although each letter of the alphabet of subungual melanoma is important, one must use all the letters together to improve early detection and thus survival of subungual melanoma. Still, as with cutaneous melanoma, the absolute diagnosis of subungual melanoma is made by means of a biopsy. (J Am Acad Dermatol 2000;42:269-74.)