Clinical-pathologic-genetic studies were performed on 3 kindreds showing the familial atypical multiple mole-melanoma syndrome (FAMMM). Findings showed vertical transmission, including father-to-son, of cutaneous malignant melanoma and/or FAMMM moles with no sex predilection. A broad spectrum of clinical signs characterizing the phenotype ranged from an apparent lack of disease expression through minimal, moderate, and florid manifestations. An extreme example was a patient with 9 separate primary melanomas in 18 years. The FAMMM moles were histologically compound nevocellular nevi with varying degrees of dysplasia of the melanocytes, an increased occurrence of fibroplasia, and chronic inflammation within the papillary dermis. Of further interest was marked variation in the degree of dysplasia in moles between and within families. These observations, when coupled with recent reports by others, are consistent with an autosomal dominant gene showing markedly variable expressivity. Management of these patients is difficult, as one cannot be certain which moles require biopsy and then, following histological study, which will require wider excision. Studies of the FAMMM syndrome should deal carefully with its natural history, including the patient's lifelong susceptibility to multiply malignant melanomas, and the possibility that cancer of other anatomic sites may be integral components of this hereditary cancer syndrome.