OBJECTIVE Our purpose was to identify factors associated with the physical activity in young children. METHODS Participants were 214 children (aged 3-5 years) enrolled in 10 childcare centers who were monitored for physical activity with an accelerometer during 2 continuous days (48 hours). Mean daily activity counts, activity counts between 9 AM and 5 PM, and percentage of time spent in vigorous activity were determined. The factors investigated were age, childcare center, season, sex, body mass index (BMI), history of preterm birth, participation in organized activities, parental BMI, and parental educational level. Regression analyses were used to identify factors associated with each of the activity measures. RESULTS A statistical model including sex, history of preterm birth, childcare center, and father's BMI accounted for 22%, 37%, and 23% of the variance in total daily counts, counts between 9 AM and 5 PM, and percentage of time spent in daily vigorous activity, respectively. Childcare center was the highest individual predictor of activity. Boys were more active than girls in all activity measures except counts between 9 AM and 5 PM. Children born preterm were less active than children born at term, and high activity levels in the child were associated with a low BMI in fathers. CONCLUSIONS The findings indicate that sex, history of preterm birth, childcare center, and father's BMI influence the daily physical activity of young children.