Proliferating cell nuclear antigen (PCNA) expression in renal cell carcinoma (RCC) was determined by immunohistochemical staining using the PC10 clone. PCNA indices ranged from 2.4 to 53.1 per cent with mean indices of 12.6, 19.0, and 31.6 per cent for grades 1 to 3 RCC and 31.9 per cent for sarcomatoid RCC. There was a significant difference between the indices of grades 1 and 3 and grades 2 and 3 tumours and between grades 1 and 2 RCC and sarcomatoid RCC. AgNOR scores and mitotic indices were determined for each tumour and comparison of PCNA indices with mean AgNOR scores and mitotic indices showed only a weak correlation (PCNA/AgNOR r = 0.406, PCNA/mitotic index r = 0.315). Tumours were divided according to PCNA index (< or = 18 per cent and > 18 per cent) and there was a significant difference in survival between the two groups, for all cases, and for each of the Robson stages. Multivariate analysis using Cox's proportional hazard model showed PCNA index, tumour stage, and mean AgNOR score to be independent predictors of survival, while tumour grade and mitotic index were found to be dependent variables.