Three groups of tumors were studied. The first group was melanomas inadvertently transmitted from donors. Brain metastases from melanoma were often misdiagnosed in the donors as primary brain tumors or cerebral hemorrhage. Eleven donors provided organs to 20 recipients of whom 3 never manifested evidence of melanoma, 1 showed local spread of tumor beyond the allograft, and 16 had metastases. Of the last group 11 died from melanoma, but 4 patients had complete remissions following transplant nephrectomy and discontinuation of immunosuppressive therapy. The second group was Melanomas treated pretransplantation. Thirty patients had cutaneous melanomas and one an ocular melanoma. Six patients (19%) had recurrences posttransplantation. Three were treated < 2 years pretransplantation, 2 between 2-5 years pretransplantation, and one 120 months pretransplantation. The third group was De novo melanomas. Cutaneous melanomas occurred in 164 patients, melanomas of unknown origin in 8, and ocular melanomas in 5. Melanomas constituted 5.2% of posttransplant skin cancers compared with 2.7% in the general population. Unusual features of cutaneous melanomas were that 6 (4%) occurred in children, and 9 (5%) occurred in bone marrow recipients who were treated for leukemia. Forty-four patients (27%) who had cutaneous melanomas also had other skin cancers. Forty-seven of 68 patients (69%) had thick skin lesions (Clark's level III or greater or > 0.76 mm by Breslow's technique). Lymph node metastases occurred in 32 patients (20%) with cutaneous melanomas. Fifty patients (30%) with cutaneous melanomas died of their malignancies, as did 5 with melanomas of unknown origin, and 1 with ocular melanoma. The risks of melanoma may be reduced by stringent selection of donors; by waiting at least 5 years between treatment of melanoma and undertaking transplantation; and, perhaps, by reducing sunlight exposure and by early excision of suspicious dysplastic lesions.