OBJECTIVE: Metformin was assessed as a treatment for weight gain in children taking olanzapine, risperidone, quetiapine, or valproate. METHOD: The subjects were 19 patients aged 10–18 years; 15 were white and four were black, and there were 12 boys and seven girls. In a 12-week open-label study, each patient received metformin, 500 mg t.i.d. Changes in weight and body mass index were evaluated by using repeated measures analysis of variance. RESULTS: Of the 19 patients, 15 lost weight, three gained 1.6 kg or less, and one had no change. The mean changes in weight and body mass index at 12 weeks were highly significant. CONCLUSIONS: Metformin merits further study as a treatment for weight gain in patients taking psychotropic medications.