OBJECTIVE: To determine the possibly greater occurrence of multiple malignancies in patients with renal cell carcinoma (RCC). PATIENTS AND METHODS: In the 7-year period 1987-93, all 1425 patients aged 15-70 years with registered histopathologically verified RCC in Norway were included in the study. All clinical and histopathology reports were checked manually, to verify the registered diagnosis and to ensure that no tumour was a metastasis from another. After this process, 257 patients (287 tumours other than RCC) with multiple primary malignancies were identified. The primary tumours other than RCC were classified as antecedent, synchronous and subsequent. For the subsequently occurring tumours, the expected number of different tumour types was calculated according to age group, gender and observation time. RESULTS: Of the 1425 patients, 228 (16%) had one, 23 (1.6%) had two, three (0.2%) had three and one (0.07%) had four other primary malignancies. In all, 100 (34.8%) of the other tumours were diagnosed as antecedent, 53 (18.7%) as synchronous and 134 (46.7%) as subsequent to the RCC. Cancer in the prostate, bladder, lung, breast, colon and rectal cancer, malignant melanomas (MM) and non-Hodgkin's lymphomas (NHL) were the most common other malignancies. The observed overall number of subsequent other malignant tumours was 22% higher than the expected number. The observed number of subsequent tumours was significantly higher for bladder cancer, NHL and MM. The estimated 15-year cumulative risk for patients with RCC and no previous or synchronous other malignancy for developing a later second cancer was 26.6% in men, and 15.5% in women (statistically significant, P = 0.04). Patients with antecedent or synchronous other cancer had significantly poorer overall survival than those without. CONCLUSIONS: Patients with RCC seem to have a significantly higher risk of developing other subsequent primary malignancies. This should be considered during the follow-up of patients with RCC.