Thirteen families have been studied clinically as the basis for a linkage study of susceptibility to cutaneous melanoma. Previous studies have shown that a number of families with predisposition to melanoma have abnormal naevi, now known as the atypical mole syndrome (AMS) phenotype. Many groups performing linkage studies using families selected from geographical areas with higher rates of melanoma have concentrated on the diagnosis of melanoma to identify presumptive gene carriers. In the UK, in the absence of extended families with multiple cases of melanoma, we have attempted to identify gene carriers through the presence of the AMS phenotype. Previously the AMS phenotype has been poorly defined and so we have developed a scoring system to define the AMS in an attempt to allow for variation in expression among gene carriers. In this report, we document the clinical characteristics of all 13 families and the use of our scoring system. The pattern of inheritance within these selected families of the AMS phenotype with or without melanoma is consistent with a dominant gene.