Given the increase in obesity in developed and developing countries and its concomitant morbidity, successful treatment approaches are needed. We examined the effect of a structured exercise intervention in overweight children in a slum in Recife, Pernambuco State, Brazil. This was a randomized, controlled efficacy trial. Seventy-eight children were randomized. Exercise was supervised, consisting of three 50' group aerobics sessions per week for six months. All participants maintained ad libitum diets. Based on intention-to-treat analyses, children in both groups had a significant increase in weight at follow-up (p-value for within-group increase < or = 0.01). The increase in weight was significantly lower in the exercise group (mean difference between groups; -1.37; 95%CI: -2.00; -0.74). A significant difference (p = 0.049) between the exercise and control groups at six-month follow-up was also found for BMI (mean difference between groups; -0.53; 95%CI: -1.06; -0.002). When we restricted the analyses to children who completed the trial (intervention = 30 and control = 38), the results were the same. An exercise program for children, sustained for six months, was effective for reducing weight gain in overweight children living in a very poor neighborhood.