Patients were investigated to gain more insight into the incidence and time course of clozapine induced weight gain (n = 81) and to compare weight gain in patients treated with clozapine (n = 31) with that of patients treated with standard antipsychotics (haloperidol, n = 11). 35.7% of the patients treated with clozapine gained weight according to our definition. If patients gained weight on clozapine this side effect was apparent within the first 12 weeks of treatment. Deviation from normal body weight at the beginning of treatment showed a significant influence on weight gain. Sex, severity of illness, comedication, mean clozapine dose and degree of improvement did not show an influence on this side effect. Weight increase was significantly higher in patients treated with clozapine than in patients treated with haloperidol.