Inadequate volumes of physical activity may be associated with the increases in inappropriately high weight in children of 5 years of age and younger. Preschools have typically lacked sufficient time in daily physical activity. Purpose A preschool-administered physical activity treatment based on social cognitive and self-efficacy theory (Start For Life) was tested for its association with increased physical activity and reduced body mass index (BMI). Methods Data from accelerometer and BMI assessments over a 9-month preschool year (18 treatment and 8 control classes of 17 to 20 children each; mean age=4.4 years) were used to contrast the behaviorally based treatment of 30 min per day with a usual care condition of the same duration. The sample was primarily African-American. Preschool teachers administered all processes. Results The Start For Life treatment was associated with a significantly greater percentage of the preschool day in moderate-to-vigorous and vigorous physical activity (approximately 30 min per week more), with sedentary time unaffected. The treatment was also associated with a significant reduction in BMI, with effect sizes greatest in overweight and obese children. (author abstract)