Seventy-three patients with primary renal neoplasms underwent kidney transplantation. Three distinct groups were identified. Thirty-four patients (group 1), who underwent antineoplastic therapy 1 year or less before transplantation, developed metastases or recurrences in 53% of the cases. In contrast, none of 15 patients in group 2 had this problem. All of these patients had a waiting period of at least 15 months between nephrectomy and transplantation. These findings emphasize the value of a lengthy waiting period between treatment of the neoplasm and performance of transplantation with its associated immunosuppressive therapy. Group 3 also had a favorable outcome. All had incidentally discovered renal malignancies, in 18 patients during the work-up of chronic renal failure or after bilateral nephrectomy in preparation for renal transplantation, and in 6 several months after transplantation when the recipient's own kidneys were removed or autopsy examination was performed. None of these 24 patients developed recurrences or metastases.