A retrospective study was done of the recurrence rates of 1297 preexisting tumors in renal transplant recipients. Of 1137 neoplasms that were treated prior to transplantation, the recurrence rate was 21%, and it was 33% in 99 cancers treated after transplantation. Fifty-four percent of recurrences in the pretransplant-treated group occurred among malignancies treated within 2 years of transplantation, 33% in those treated 2-5 years before transplantation, and 13% among those treated more than 5 years pretransplantation. Among the 31 neoplasms in the last group 52% of recurrences occurred within 2 years after transplantation. Among those cancers treated pretransplantation the highest recurrence rates occurred with breast carcinomas (23%), symptomatic renal carcinomas (27%), sarcomas (29%), bladder carcinomas (29%), nonmelanoma skin cancers (53%) and multiple myeloma (67%). In the tumors treated posttransplantation 39% of recurrences were from nonmelanoma skin cancers. The bulk of evidence suggests that immunosuppressive therapy facilitates the growth of residual cancers. With some exceptions a minimum waiting period of 2 years between treatment of a neoplasm, with a favorable prognosis, and undertaking renal transplantation is desirable. A waiting period of approximately 5 years is desirable for lymphomas, most carcinomas of the breast, prostate or colon, or for large (> 5 cm) symptomatic renal carcinomas. No waiting period is necessary for incidentally discovered renal carcinomas, in situ carcinomas, and possibly tiny focal neoplasms. As it is highly unlikely that most candidates for nonrenal transplantation can be kept alive for a two year waiting period nonrenal transplantation can be undertaken in patients who have been treated for major cancers, provided that the disease appears to have been adequately controlled, and that the stage of the malignancy does not have a poor prognosis.