The authors studied weight gain mechanisms and energy balance in patients treated with olanzapine. The body mass index of male schizophrenic adolescent inpatients treated with olanzapine (N=10) and of 10 matched patients treated with haloperidol (N=10) were measured at baseline and after 4 weeks of treatment. For the patients treated with olanzapine, caloric intake, resting energy expenditure, and physical activity (determined through accelerometry and heart rate monitoring) were assessed at baseline and after 4 weeks of treatment. Body mass index significantly increased in those treated with olanzapine but not in those given haloperidol. The increase in body mass index was due to an increase in caloric intake without change in diet composition. Olanzapine had no significant effect on resting energy expenditure. Daily energy expenditure was very low before and after treatment. Olanzapine-induced weight gain is associated with a general increase in caloric intake.