The purpose of this study was to test the utility of a new instrument, the Child/Adolescent Activity Log (CAAL), designed to measure children's physical activities in a school setting efficiently, yet comprehensively. At baseline, 459 participants (mean age 12 + 1.4, 59% White, 34% African American, 7% other) were recruited to participate in a 2-year study focusing on patterns and predictions of physical activity at critical school transition points. The CAAL was administered daily for a 1-week period on 6 occasions to this cohort. From the CAAL, information about the pattern of specific activities chosen, average daily duration of activity, and average daily expenditure per kg body weight was obtained. The validity of the log was supported by its (1) correlation with Caltrac readings, (2) relation in the predicted direction with a single-item measure of typical level of physical activity and fitness indices, and (3) expected changes in exercise patterns over time and by gender. If future studies corroborate the psychometric properties and ease of administration of the CAAL, its utility in community-based studies is promising.