Limited information is available about gender and ethnic differences in red cell distribution width (RCDW) with regard to its relation to mortality in a population free of cardiovascular (CV) disease and diabetes. To assess gender and ethnic differences in RCDW and their effect on the association between RCDW and mortality, the Third National Health and Nutritional Examination Survey (n = 15,460, 1988 to 1994) data were examined. Multivariate adjusted Cox proportional hazard analysis was performed to assess effect of gender and ethnicity on the association between RCDW and mortality (total, CV disease, and coronary heart disease [CHD]). RCDW (mean ± SE) was greater in black women (13.1 ± 0.03) and men (13.4 ± 0.02) compared to women of white (12.9 ± 0.02) and other (13.0 ± 0.07) ethnicities and men of white (13.3 ± 0.02) and other (13.3 ± 0.07) ethnicities, respectively (p <0.001). The interaction between RCDW and gender was statistically significant for all study outcomes (p <0.001) but nonsignificant for RCDW and ethnicity. After adjusting for key variables, RCDW in women was associated with adjusted hazard ratios of 1.22 (95% confidence interval [CI] 1.14 to 1.31) for all-cause mortality, 1.17 (95% CI 1.07 to 1.28) for CV deaths, and 1.18 (95% CI 1.03 to 1.35) for CHD deaths; in men, adjusted hazard ratios were 1.29 (95% CI 1.20 to 1.38) for all-cause mortality, 1.27 (95% CI 1.17 to 1.37) for CV deaths, and 1.25 (95% CI 1.13 to 1.39) for CHD deaths (p <0.05 for all). In conclusion, blacks and men have significantly greater RCDWs compared to whites and women. Greater RCDW is associated with a greater risk of mortality in men compared to women, whereas no effect modification is observed by ethnicity.