Between December 1968 and July 1995, 2372 renal transplant recipients were seen regularly in our outpatient clinic. 154 patients (6.5%) of these subsequently developed a malignant tumor. One of the most frequently observed malignancies was a renal cell carcinoma in the native kidneys, found in 12 patients (7.8%; i.e. 0.5% of the total population), two of them with analgesic nephropathy and one with adult polycystic kidney disease. So-called acquired cystic kidney disease of the native kidneys was found in 73% of the patients with renal cell carcinoma. In the patients with renal cell carcinoma the median age at transplantation was 43.9 (39-55) yr. The median time from onset of end-stage renal failure and from transplantation to the appearance of the tumor amounted to 117.3 (43-206) and 75.8 (5-164) months, respectively. Despite nephrectomy, four patients died of the cancer (6-57 months). As a consequence we suggest a regular sonographic screening of the native kidneys twice a year in all renal transplant patients. As in dialysis patients, the main risk factors for the development of renal cell carcinoma seem to be so-called acquired cystic disease of the native kidneys and the underlying renal disease, such as analgesic nephropathy, and obviously not the immunosuppressive regimen after transplantation.