Validity and reliability of a short physical activity history were assessed in two studies. Validity was studied in 2766 women and 2303 men, participants in CARDIA, a biracial study. Ages ranged from 18 to 30 years. The activities performed in the past 12 months by ≥ 50 per cent of participants were walking/hiking, nonstrenuous sports, shoveling/lifting during leisure, running/jogging and home maintenance/gardening. Validity was indirectly assessed by studying the relationships of total activity to skinfold thickness, total caloric intake, duration on a self-limited maximal exercise test, and high density lipoprotein cholesterol. Less than perfect correlations are expected since physical activity is not the only factor affecting the validation criteria and since physical activity patterns change over time within each person. Comparing the highest physical activity quartile to the lowest physical activity quartile, mean level of sum of three skinfolds was 10.7 mm less for women (correlation coefficient (r) = -0.15, P < 0.001) and 6.9 mm less for men (r = -0.12, P < 0.001); mean level of caloric intake was 158 kcal more for women (r = 0.07, P < 0.001) and 875 kcal more for men (r = 0.21, P < 0.001); mean level of duration on treadmill was 132 seconds more for women (r = 0.36, P < 0.001) and 95 seconds more for women (r = 0.25, P < 0.001); and mean level of high density lipoprotein cholesterol was 4.8 mg/dL more for women (r = 0.13, P < 0.001) and 3.2 mg/dL more for men (r = 0.11, P < 0.001). Reliability was studied in a separate population by comparing questionnaire results in an initial telephone administration with results obtained two weeks later (N = 129). Similar types and amounts of activity were reported in this group as in the group studied for validity. Test-retest correlation coefficients for three summary scores ranged from 0.77 to 0.84, and were at least 0.57 for each of the 13 activity groupings queried. This questionnaire typically takes 5-10 minutes to administer. It yields moderately detailed information about type and amount of usual leisure time physical activity.