OBEJECTIVE: To examine the relative size of the effects of fitness and fatness on mortality in Russian men, and to make comparison to US men. DESIGN: Prospective closed cohort. SUBJECTS: 1359 Russian men and 1716 US men aged 40-59y at baseline (1972-1977) who were enrolled in the Lipids Research Clinics Study. MEASUREMENTS: Fitness was assessed using a treadmill test and fatness was assessed as body mass index (BMI) calculated from measured height and weight. Hazard ratios were calculated using proportional hazard models that included covariates for age, education, smoking, alcohol intake and dietary keys score. All-cause and cardiovascular disease (CVD) mortality were assessed through 1995. RESULTS: In Russian men, fitness was associated with all-cause and CVD mortality, but fatness was not. For mortality from all causes, compared to the fit-not fat, the adjusted hazard ratios were 0.87 (95% Cl: 0.55, 1.37) among the fit-fat, 1.86 (95% Cl: 1.31, 2.62) among the unfit-not fat and 1.68 (95% Cl: 1.06, 2.68) among the unfit-fat. Among US men, the same hazard ratios were 1.40 (95% Cl: 1.07, 1.83), 1.41 (95% Cl: 1.12, 1.77) and 1.54 (95% Cl: 1.24, 2.06), respectively. There were no statistically significant interactions between fitness and fatness in either group of men for all-cause or CVD mortality. CONCLUSION: The effects of fitness on mortality may be more robust across populations than are the effects of fatness.