Most epidemiologic studies in nutrition have concentrated on body fat and obesity because of associated health risks, while few studies have examined factors that influence body cell mass (BCM). The relative influences of sex, race, environment, and age upon BCM were compared by analyzing the results of bioimpedance analyses in two cohorts of 1094 healthy adults, including Africans in Zaire, plus African Americans and Caucasians in New York City. Men were taller, heavier, and had a larger BCM and fat-free mass (FFM) than women, while women had more fat than men. African American men and women had more BCM, FFM, and fat than Africans. In contrast, BCM and FFM were not different in African Americans and Caucasians, and body fat was higher only in African American than in Caucasian women. Sex influenced the effects of environment and race, since the majority of the weight differences in men were in FFM, while the majority of the weight differences in women were in fat. The effect of sex upon BCM was stronger than the effects of environment (p < 0.001) or race (p < 0.001), and the effect of environment was stronger than the effect of race (p = 0.012), so that the relative strengths were sex > environment > race. Race had a stronger effect upon FFM than upon BCM. Since race did not affect BCM significantly, it may affect other components of FFM, e.g., extracellular water or solids, such as skeletal mass. The results demonstrate that sex affects normal body composition to a greater degree than race or environment. Am. J. Hum. Biol. 10:259-268, 1998. ยฉ 1998 Wiley-Liss, Inc.