Eight second malignant tumours developed in a population-based series of 218 patients diagnosed with renal tumours in childhood: renal cell carcinoma of the contralateral kidney, hepatocellular carcinoma, Hodgkin's disease, and 4 basal cell and 1 squamous cell carcinomas of skin. Excess risk of developing a second malignancy (excluding skin carcinomas but including a registrable spinal neurofibroma) was 14.7 (95% CI 4.0-37.7, P = 0.0003) for Wilms' tumour patients. Cumulative incidence of second malignant neoplasms (excluding skin carcinoma) was zero at 10 years, 5.0% at 20 years, and 10.2% at 30 years. The most common second neoplasms seen were benign osseous/chondromatous tumours and 4 of the 7 Wilms' tumour patients with malignant tumours had previous or synchronous tumours of this kind. Development of bony exostoses may be a marker for those patients at particularly high risk of subsequent malignancy.