Data regarding posttransplant cancers are reviewed from the Cincinnati Transplant Tumor Registry (CTTR) and from the literature. The CTTR has data on 9,688 types of cancer that developed in 9,032 renal allograft recipients. The predominant tumors are lymphomas and lymphoproliferations (PTLD), carcinomas of the skin and lips, carcinomas of the vulva and perineum, in situ carcinomas of the cervix of the uterus, Kaposi's sarcoma (KS), hepatocellular carcinomas, renal carcinomas, and various sarcomas (excluding KS). Prominent features of the PTLD cases are their high incidence, frequent involvement of extranodal sites, a marked predilection for the brain, and frequent involvement of the allograft by tumor. Skin cancers also present unusual features, a remarkable high frequency of KS, reversal of the ratio of basal cell carcinomas to squamous cell carcinomas that is seen in the general population, the young age of the patients, the high incidence of multiple tumors (in 44% of patients), and the aggressive behavior of some squamous cell carcinomas. Cancers of the vulva and perineum occur at a much younger age than in the general population and are preceded by a history of condyloma acuminatum in over 57% of cases. Reduction or cessation of immunosuppressive therapy is of value in some patients with PTLD or KS but carries the risk of allograft rejection.