DNA aneuploidy has repeatedly been shown to be a significant prognostic indicator in renal cell carcinoma; however, few studies have emphasized the importance of measurements of cellular proliferation. This study evaluated 55 patients treated by radical nephrectomy and for whom clinical follow-up was available. There were 36 men and 19 women with a mean age of 61 yr. Robson stage distribution was I, 38 cases; II, five cases; and III, 12 cases. Flow cytometric analysis in 44 cases revealed 29 DNA diploid and 15 DNA aneuploid tumors with a median synthesis-phase fraction (SPF) of 4.4% (range 1.0 to 31.4%). The median proliferating cell nuclear antigen index was 3.9% (range 0.1 to 58.0%). There was a significant correlation between SPF and proliferating cell nuclear antigen index (R = 0.769) in the 33 cases in which both were available. Cellular proliferation, as determined by SPF, was a significant prognostic indicator (P < 0.02), but proliferating cell nuclear antigen index did not correlate with outcome (P > 0.05). Other significant predictors in this study were Robson stage (P = 0.03) and nuclear grade (P = 0.0003). DNA ploidy did not correlate with outcome (P > 0.05). We conclude that cellular proliferation, as measured by SPF analysis, is a significant predictor of outcome for renal cell carcinoma. Although the proliferating cell nuclear antigen index correlated with SPF, it did not achieve statistical significance as a prognostic indicator.