Cancer incidence in transplant patients ranges from 4% to 18% (average, 6%). We present data on 7668 types of malignancy that occurred in 7192 patients. The predominant tumors are skin/lip carcinomas, lymphomas, vulvar/perineal carcinomas, in situ uterine cervical carcinomas, Kaposi's sarcoma (KS), renal carcinomas, hepatobiliary tumors, and soft tissue/visceral sarcomas. Tumors appear relatively early after transplantation. The earliest is KS, appearing an average of 22 months post-transplantation whereas the latest are vulva/perineal carcinomas, presenting an average of 113 months posttransplantation. Skin cancers present unusual features: a remarkable frequency of KS; reversal of the high ratio of basal to squamous-cell carcinomas seen in the general population; the low age of the patients; and a high incidence of multiple tumors (in 42% of patients). Unusual features of lymphomas are the high incidence of non-Hodgkin's lymphoma (NHL); common involvement of extranodal sites; marked predilection for the brain; and frequent allograft involvement. Lymphomas are much more common in nonrenal than in renal allograft recipients. Vulvar/perineal carcinomas occur at a much lower age than in the general population, and are often preceded by condyloma acuminatum. Neoplastic regression following reduction or cessation of immunosuppressive therapy occurs in some patients with NHL or KS.