Objective: The objective of this study was to investigate odds ratios of overweight/obesity in children with ADHD and to compare the change in body mass index (BMI) after initiation of methylphenidate treatment in normal versus overweight/obese children. Method: This population-based study included 724 children (<18 years), of whom 197 were girls. Odds ratios for overweight and obesity were calculated, comparing the study group with a reference group from the same area. After initiation of methylphenidate treatment, changes in BMI were assessed for up to 3 years. Results: Children with ADHD had an odds ratio of 1.87 (95% confidence interval [CI]: [1.60, 2.19]) for overweight/obesity. A decrease in BMI standard deviation score was identified 1 to 3 years into treatment. The decrease was beneficially greater in overweight/obese as compared with normal weight children-mean (SD) -0.64 (0.80) versus -0.39 (0.68); p = .001-and greater in girls. Conclusion: Medication with methylphenidate may facilitate favorable weight development in children with ADHD and overweight/obesity.