To investigate the effect of obesity at diagnosis on prognosis of renal cell carcinoma, 360 renal cell carcinoma patients newly diagnosed at 29 hospitals in Oklahoma between January 1, 1981 and December 31, 1984 were followed through December 31, 1987. The Cox proportional-hazard model was used to estimate the hazard ratio, adjusting for other potentially prognostic factors. Both the disease-free interval and the overall survival were longer in patients who were obese (greater than or equal to 120% standard body mass index) at diagnosis. The adjusted-hazard ratio for disease recurrence between obese and nonobese patients was 0.43 (95% confidence interval [CI], 0.19 to 0.98). The obese patients had an adjusted death hazard rate 0.68 times that of the nonobese patients (95% CI, 0.38 to 1.22). Although obesity was reported to increase the risk for renal cell carcinoma, prognosis was no worse and may be better among obese patients with the disease.