An immunocytochemical and cytophotometrical analysis of 30 renal cell carcinomas was carried out in order to investigate the expression of cytokeratin and vimentin in relation to the ploidy and mitotic index. Thirty renal cell carcinomas were studied using monoclonal antibodies CAM 5.2 and anti-vimentin in a biotin-streptavidin staining procedure. The renal cell carcinomas were classified according to the criteria of Fuhrman with a nuclear grading from I to IV. All carcinomas expressed cytokeratin, whereas co-expression with vimentin was present only in 53%. Expression for both intermediate filaments was present in aneuploid tumours with a marked nuclear pleomorphism and was more frequently related to a tubulo-papillary and pseudosarcomatous growth pattern. It is suggested that vimentin expression in renal cell carcinomas may be useful as a prognostic index in addition to nuclear grade. DNA content, mitotic rate and tumour stage.