The prevalence of obesity increased in the United States through the 1980s. The authors examined 10-year aging and secular (time-related) trends in the Coronary Artery Risk Development in Young Adults (CARDIA) cohort for indications of whether these trends are continuing and for ages of peak weight gain in young adults. CARDIA is a population-based, prospective study of 5,115 African-American and White men and women aged 18-30 years at baseline. Body weight and overweight prevalence were measured at five time points from 1985-1986 to 1995-1996. Linear, mixed-model regression was used to partition weight gain into that due to secular trends and that due to aging. Prevalence of overweight (body mass index (BMI) > or = 25.0 kg/m2) increased markedly, and prevalence of severe obesity (BMI > or = 40.0 kg/m2) doubled in all race-sex groups. Each race-sex group experienced significant secular weight gains, ranging from 0.96 kg/year (95% confidence interval: 79, 1.13) in African-American women to 0.55 kg/year (95% confidence interval: 0.41, 0.69) in White women. Significant secular gains were present during each follow-up period. Each race-sex group also experienced significant weight increases related to aging during their early to midtwenties. Secular trends for weight gain are continuing in CARDIA, but the magnitude of weight gain differed among the four race-sex groups.