Recent trends toward obesity and associated health risks in children highlight the significance of weight gain as a side effect with certain antiepilepsy drugs. No previous study has prospectively compared, in adolescents, weight effects for two commonly used antiepilepsy drugs. We report results from a post hoc subanalysis of adolescent data from a randomized, double-blind study comparing weight effects of lamotrigine and valproate. Patients were > or = 12 years of age with new-onset partial or generalized seizures who were randomized 1:1 to lamotrigine or valproate. Patients were escalated to a dose range of 100 to 500 mg/day for lamotrigine and 10 to 60 mg/kg/day for valproate based on clinical response, with target doses maintained for 24 weeks. Results are reported for adolescents aged 12 to 20 years. Weight changes during maintenance were higher (P < .05) in valproate (n = 20) patients than in lamotrigine (n = 18) patients, and change in body mass index was higher (P < .05) in valproate patients at the end of the study. At week 32, mean body mass index in the valproate group was above the 85th percentile representing "at risk for overweight." Whereas weight remained stable in adolescents treated with lamotrigine, weight increased in those treated with valproate by week 10 of this study and continued to increase at the end of the study.