A cohort of 1245 adolescents, 12-16 yr old and participating in a prospective study of risk factors for injury, were surveyed annually to assess past year leisure physical activity. This report describes the development and reproducibility of the questionnaire and provides the descriptive epidemiology of leisure physical activity in adolescents. The questionnaire was self-administered twice, a year apart, and had a 1 yr test-retest rank order correlation of 0.55. The past year estimate of leisure activity was also shown to be related to fitness (rho = -0.37), which was defined as the time needed to complete a 1-mile run. Males reported significantly more activity than females (P < 0.05) and whites reported more activity than nonwhites (P < 0.05). However, socioeconomic status was not found to be a determinant of activity levels in either males or females. In females, a negative association between activity and age was found (P < 0.05); however, this association was not evident in males. In summary, an activity questionnaire has been developed and was shown to be both reproducible and feasible. Therefore, it was used to examine habitual leisure physical activity patterns of adolescents.