Aim Treatment of childhood obesity is often insufficient and may be aggravated by high co-occurrence of attention-deficit/hyperactivity disorder (ADHD). We aimed to investigate whether children with overweight or obesity normalised in weight when receiving stimulant treatment for ADHD. Methods Growth data of 118 children were obtained from medical records at outpatient paediatric and children’s psychiatric services in the Gothenburg area, Sweden. The children were diagnosed with ADHD and were between 6 and 17 years at the start of stimulant treatment. The pre-treatment data act as an internal control where every child is their own control. Results At the start of treatment, 74 children had normal weight and 44 had either overweight or obesity. During the year with stimulants, the mean (SD) body mass index (BMI) in standard deviation score (SDS) decreased significantly: -0.72 (0.66) compared with 0.17 (0.43) during the year before treatment (p < 0.01). After one year with treatment, 43% of those with overweight or obesity had reached normal weight. Conclusions Stimulant treatment for ADHD yields significant weight loss. In children with overweight or obesity and ADHD, this is an important finding showing additional benefit in terms of weight management.