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This study assessed the reliability and validity of the Previous Day Physical Activity Recall (PDPAR), a self-report instrument designed to measure physical activity in youth. Subjects were randomly selected students in grades 7-12. Test-retest reliability was reported as the correlation between estimated relative energy expenditures determined from two PDPAR administrations completed within 1 h. Interrater reliability was determined by two observers using the scoring protocol. Validity was assessed using footstrikes (pedometer), Caltrac activity counts, and heart rate monitoring as criterion measures. Interrater and test-retest reliability was 0.99 and 0.98, respectively (P < 0.01). The correlation between relative energy expenditure from the PDPAR (kcal·kg-1l·d-1) and pedometer and Caltrac counts was 0.88 (P < 0.01) and 0.77(P < 0.01), respectively. The correlation between percentage heart rate range (HRmax-HR-rest) and mean energy expenditure from the PDPAR was 0.53 (P < 0.01). The correlation between 1-min heart rates > 50% HRR sustained for 20 min and the number of 30-min blocks with a relative energy expenditure of at least four metabolic equivalent tasks(MET) was 0.63 (P < 0.01). The PDPAR provides valid and reliable estimates of physical activity and also accurately identifies bouts of moderate to vigorous activity.