OBJECTIVE: To assess physical activity and energy expenditure and to identify their correlates during adolescence and early adulthood. DESIGN: In a cohort study, total energy expenditure (TEE) and physical activity level (PAL) were assessed at 15 and 21 y from a 7-day activity diary and predicted BMR. The influences on TEE and PAL of body size, gender, sexual maturity, physical activity habits, sitting time, region, social conditions, employment, food habits, smoking and alcohol intake were examined in multivariate analyses.Subjects:71 male and 89 female subjects, living in two socioeconomically different regions of Sweden, a university region and an industrial region. SETTING: The university city of Uppsala and the industrial town of Trollhättan. RESULTS: At both 15 and 21 y, PAL and TEE were high, with gender, sitting time and physical activity habits as main correlates. At age 21 y, employment and the mother's educational level also appeared as significant correlates. The relations between the main variables and their correlates were more complex than at age 15 y, and the sitting time was reduced and the activity habits were changed. CONCLUSIONS: A reduction of daily sitting appears to be a major reason why high levels of physical activity and energy expenditure were maintained from 15 to 21 y of age in spite of changed and less frequent activity habits during this interval.