A study on cancer incidence after kidney transplantation was performed using data of national transplant and cancer registries. Since 1964 up to 30 June 1997, 3440 kidney transplantations were performed on 2890 patients. From 1967 to 1997, 230 posttransplantation malignancies were found in 20,817 patient-years of follow up. The standardised incidence ratio (SIR) was 3.33 compared to the general population. The SIR was highest in skin cancer (39.2). The SIRs were high in cancers of the lip (23.0), thyroid (8.08), kidney (8.0), lower urinary tract (3.2), non-Hodgkin lymphoma (4.8), ovary (3.9) and colon (3.9). Skin cancer and lymphomas had much higher SIRs in men than in women whereas lower urinary tract cancer had a higher SIR in women. During the first 10 follow up years, life-table analysis indicates a higher cancer risk in cyclosporine-treated patients, but this may be biased by their shorter follow up as the overall SIR was equal in both groups. This population study shows the increased incidence of cancer in the transplant population and points out the importance of cancer surveillance in the years following kidney transplantation.