This study of 939 pre-existing malignancies that occurred in 913 renal transplant recipients showed that in 823 patients the tumors were treated prior to or at transplantation, in 78 after transplantation, at an unspecified time in 20, while 18 received no treatment. Of patients treated pretransplantation 185 (22%) developed recurrences posttransplantation. Low recurrence rates (0-10%) occurred with incidentally discovered renal tumors; lymphomas; and testicular, uterine cervical, and thyroid carcinomas. Intermediate recurrence rates (11-25%) occurred with carcinomas of the uterine body; Wilms' tumors; and carcinomas of the colon, prostate, and breast. High recurrence rates (> or = 26%) occurred with carcinomas of the bladder, sarcomas, malignant melanomas, symptomatic renal carcinomas, nonmelanomatous skin cancers, and myelomas. Overall 53% of 185 recurrences occurred in patients treated 0-24 months pretransplantation, 34% in patients treated 25-60 months pretransplantation, and 13% in patients treated > 60 months pretransplantation. Of 78 patients whose cancers were first treated after transplantation, 27% developed recurrences. However, 63% did not do so in follow-ups averaging 53 months. A two-year waiting period between treatment of cancer and transplantation is justified for most neoplasms except for incidentally discovered renal carcinomas, in situ carcinomas, and possibly focal neoplasms (a small single focus), low-grade bladder cancers, and basal cell skin cancers. In these cases no waiting period is necessary. On the other hand, a waiting period > 2 years is necessary for most malignant melanomas, breast carcinomas, and colorectal carcinomas. Conflicting data are presented as to whether immunosuppression affects growth of existing tumor cells but most of the evidence suggests acceleration of neoplastic growth.