This study examined the effect of preschool physical activity on the change in body fatness from preschool to first grade. The Framingham Children's Study, a longitudinal study of childhood cardiovascular risk behaviors, began in 1987 with the enrollment of 106 children aged 3-5 years and their parents. The present analyses include 97 healthy children with complete data from study entry into first grade. Physical activity was assessed twice yearly for 5 days with an electronic motion sensor. The authors estimated change in the child's level of body fat from preschool to first grade by using the slopes of triceps and subscapular skinfolds and body mass index. On average, active girls (i.e., those with above-median activity levels) gained 1.0 mm in their triceps skinfolds from baseline to first grade, while inactive girls gained 1.75 mm. Active boys lost an average of 0.75 mm in their triceps, while inactive boys gained 0.25 mm. When age, television viewing, energy intake, baseline triceps, and parents' body mass indices were controlled for, inactive preschoolers were 3.8 (95% confidence interval 1.4-10.6) times as likely as active preschoolers to have an increasing triceps slope during follow up (rather than a stable or decreasing slope). This relative risk estimate was slightly higher for children with more body fat at baseline. In this study, preschool-aged children with low levels of physical activity gained substantially more subcutaneous fat than did more active children.