The associations of body size with age, race, sex, and education in young adults were examined in 5,115 black and white, men and women ages 18-30 years. Black women were more obese than white women with greater mean levels of body mass index (25.8 vs 23.1 kg/m2), subscapular skinfold thickness (19.9 vs 15.2 mm), and waist girth (76.7 vs 72.0 cm), all P less than 0.0001. Black women were more likely to exceed 20% of ideal body weights (black women 23.7%, white women 9.1%, P less than 0.0001). No similar differences were found in men. Associations of measures of body size with age and education differed among race/sex groups. Body mass index and skinfolds increased with age among white and black men and black women, but not among white women. The association of education with obesity was negative among white women and positive among black men with no significant association noted among white men and black women. These data show a complex relationship between age, sex, race, education, and obesity in young adulthood.