BACKGROUND: There is little information about validation of young children's self-reported physical activity. This study assessed the validity of a self-reported questionnaire designed to measure children's physical activity. METHODS: Subjects were 34 boys from 4th to 6th grade of a public elementary school. Contents of the self-reported physical activity questionnaire included participation in sports club, physical activity intensity, preference for physical activity, and frequency of physical activity. Subjects were equipped with a Lifecorder and an Actiwatch for 7 consecutive days to monitor physical activity. Physical activity index was calculated from the Lifecorder data of total energy expenditure per day (TEE) divided by basal metabolic rate. Unpaired t-test, analysis of variance, and multiple linear regression analysis were performed to clarify the relationship between the objective and subjective data. RESULTS: Subjects who reported participation in a sports club had a higher physical activity index and energy expenditure originating from physical activity than those who did not. Those characterized by a "vigorous" physical activity intensity had a higher physical activity index (1.63_??_0.08), when compared with peers in "moderate" (1.59_??_0.06) or "light" categories (1.54_??_0.07) (p for linear trend p<0.05). A high frequency of physical activity was significantly associated with an increasing trend in energy expenditure originating from physical activity, steps, and activity counts. Preference for physical activity was significantly related to data from the Lifecorder and the Actiwatch. CONCLUSIONS: Schoolchildren's self-reported physical activity is in accordance with the objective data, and could be used as a valid measure to evaluate physical activity level in school settings. J Epidemiol 2003;13:278-287.