A study was undertaken to investigate the development of carcinoma in patients' own kidneys after renal transplantation. Twenty carcinomas were diagnosed among 16,755 patients grafted from 1952 to February 1993. It was possible to collect data for 17 detected carcinomas. These tumours developed in 17 patients, 14 male and 3 female, aged 31 to 64 years old. They appeared an average of 40 (range 1-204) months after transplantation. The maintenance treatment consisted of cyclosporine in 15 recipients. Four patients demonstrated clinical signs. The other 13 carcinomas were diagnosed as an incidental finding on ultrasound (n = 10), at autopsy (n = 1) or by examination of kidneys from nephrectomy (n = 2). In the patient group with incidental diagnosis, tumors were larger than 4 cm in 3 out of the 9 cases studied; they were confined to the kidney in 5 cases and lymph node invasion or renal vein involvement were noted in 1 case. Distant metastases were present in 3 symptomatic patients and in 1 "incidental" case. Except for tumours discovered on nephrectomies or autopsy, a nephrectomy was performed in all cases. Death occurred 1 to 12 months after diagnosis in the recipients with metastatic tumours. A colic carcinoma was diagnosed 6 months after nephrectomy in one patient. The other 11 patients are doing well as the immunosuppressive treatment is being continued.