Obese children 8-12 years old from 61 families were randomized to treatment groups that targeted increased exercise, decreased sedentary behaviors, or both (combined group) to test the influence of reinforcing children to be more active or less sedentary on child weight change. Significant decreases in percentage overweight were observed after 4 months between the sedentary and the exercise groups (-19.9 vs. -13.2). At 1 year, the sedentary group had a greater decrease in percentage overweight than did the combined and the exercise groups (-18.7 vs. -10.3 and -8.7) and greater decrease in percentage of body fat (-4.7 vs. -1.3). All groups improved fitness during treatment and follow-up. Children in the sedentary group increased their liking for high-intensity activity and reported lower caloric intake than did children in the exercise group. These results support the goal of reducing time spent in sedentary activities to improve weight loss.