Physical activity was assessed by questionnaire among 4,956 young blacks and whites aged 18-30 years at the baseline examination (1985-1986) of the Coronary Artery Risk Development in Young Adults (CARDIA) study, a longitudinal study of cardiovascular risk factors. The Physical Activity Recall questionnaire categorized all activity during the previous week, while the Physical Activity History questionnaire quantified participation in 13 specific activities during the previous year. This report compares the two questionnaires with regard to their characterization of the activity levels of the sociodemographic subgroups of the study population and their associations with known physiologic correlates of physical activity. Both questionnaires resulted in the same physical activity patterns for sex (men greater than women) and age (younger greater than older) strata. However, the mean Physical Activity History score was higher in white women than in black women, while the Physical Activity Recall scores were nearly equal. The Physical Activity History score was directly related to educational status, and the Physical Activity Recall score was inversely related to educational status. The Physical Activity History score was generally more strongly associated with physiologic variables known to be related to physical activity (e.g., treadmill test duration). Based upon these findings, which may only be appropriate in this age group, it was concluded that the Physical Activity History score was the more valid measure of habitual physical activity in this study group of young adults.