Origin of cutaneous melanoma in a congenital dysplastic nevus spilus.

Cutaneous melanoma developed in contiguity with a congenital nevus spilus on the leg of a 79-year-old white woman. The unique features of the nevus spilus in this case were its relatively large size (diameter, 8 cm), irregular gross appearance, lifelong stability until the recent appearance of a tumor nodule, and the presence of intraepidermal melanocytic dysplasia appearing as multifocal elements within darkly pigmented speckles distributed throughout a lightly pigmented background of lentigo simplex. Based on this observation, we suggest that the presence of intraepidermal melanocytic dysplasia in nevus spilus may be a predisposing factor for the development of melanoma. The malignant potential of "dysplastic" nevus spilus requires further study.