Technical Abstract: Our objective was to compare the differential efficacy of a weight loss program for Mexican-American children who are overweight, obese, and severely obese. Study participants were enrolled in an intensive weight loss intervention aimed at improving eating and physical activity behaviors with behavior modification strategies. Participants included 212 children (45% female) between the ages of 9 and 14 (mean = 12.0, standard deviation = 0.7). All participants were classified as overweight, obese, or severely obese. Repeated measures analyses revealed that children in the overweight, obese, and severely obese weight categories differed significantly in standardized body mass index (zBMI) decreases at baseline, 3, 6, and 12 months (F = 4.57, P < .01, partial eta * 2 = .06). Follow-up paired sample tests showed a significant change in zBMI from baseline to 3 and 6 months for children in the overweight, obese, and severely obese weight categories. However, at 12 months only the overweight and obese students continued to show significant improvement from baseline in zBMI. In conclusion,these findings suggest that an intensive behavioral weight loss intervention that has demonstrated efficacy for decreasing zBMI may have incrementally smaller effects for children as weight classification increases.