Objectives. This study estimated the amount of weight change in a biracial cohort of young adults and the separate components attributable to time-related and aging-related changes, as well as identified possible determinants of weight change. Methods. In this population-based prospective study of 18- to 30-year-old African-American and White men and women, body weight and prevalence of overweight were measured from 1985/86 to 1992/93. Results. Average weight increased over the 7 years, increases ranging from 5.2 kg (SE = 0.2, n = 811) in White women to 8.5 kg (SE = 0.3, n = 882) in African-American women. Significant time-related increases in weight, ranging from 2.0 kg (SE = 1.0) in White women to 4.8 kg (SE = 1.0, n = 711) in African-American men, accounted for 40% to 60% of the average total weight gain. Aging-related increases were also significant, ranging from 2.6 kg (SE = 0.8, n = 944) in White men to 5.0 kg (SE = 1.1) in African-American women. The prevalence of overweight increased progressively in each group. Decreased physical fitness was most strongly associated with weight gain in both sexes. Conclusions. The observed dramatic time-related weight gains, most likely due to secular (period-related) trends, are a serious public health concern.