AIMS--To perform stereological quantitation of volume weighted mean nuclear volume in renal cell carcinomas; and to correlate the data obtained with recognised clinical and pathological variables and determine their prognostic value. METHODS--The point-sampled intercepts method was used to estimate mean nuclear volume in 63 cases of clear cell renal carcinoma diagnosed between 1980 and 1988. New paraffin wax embedded histological sections were analysed after systematic sampling and the test systems superimposed on a projected microscopic image to measure nuclear intercept lengths. After mathematical estimation of mean nuclear volume, statistical analyses of the data in relation to clinical and pathological variables as well as the prognostic impact were investigated. RESULTS--The mean nuclear volume was significantly associated with tumour dedifferentiation. However, mean nuclear volume showed no statistical differences with sex, age, and clinical stage. The prognostic value of mean nuclear volume, nuclear grading, and clinical stage in renal cell carcinomas was high: mean nuclear volume greater than 140 micron3 was associated with short term survival. CONCLUSIONS--Measurement of mean nuclear volume was useful as a guide to objective grading of renal cell carcinomas, though there was an overlap between tumour grades. Based on the limited number of cases analysed, the mean nuclear volume is proposed as an additional prognostic indicator.