This study comprises five cancer prone families with one or more occurrences of melanoma in context with a variety of other histologic types of cancer, including an excess of multiple primary malignant tumors. The genetic mode of transmission of cancer in each family was consistent with a dominant factor which has seemingly predisposed these patients to a general cancer diatheses. The tumor spectrum included malignant melanoma and cancers of the breast, gastrointestinal tract, lung and lymphoproliferative system. Recently, S1 and S2 antigens were noted to occur with malignant melanoma and the same variety of tumors which have occured in these five families.