The Mosan Study Group of Pigmented Neoplasms groups 9 dermatopathologists and pathologists from 9 laboratories. We collect information concerning the epidemiology and the histological nature of malignant melanomas and of their precursors in a circumscribed part of Belgium. During the past three decades there has been a steadily increasing number of primary cutaneous malignant melanomas other than those developed on lentigo maligna. Women are twice more often affected than men. Malignant melanomas supervening on a preexisting melanocytic nevus are of rare occurrence.