The aim of this study was to evaluate the effect of long-term clozapine treatment on body weight changes in neuroleptic-resistant chronic schizophrenic patients and to compare it with that of classical antipsychotic agents. The body mass index (BMI) of 96 neuroleptic-resistant chronic schizophrenic patients was calculated before the beginning and after long-term (mean +/- SD 1.7 +/- 1.3 years) clozapine treatment. These data were compared to the BMI of 98 chronic schizophrenic patients maintained on classical antipsychotic agents for a similar duration (mean +/- SD 1.9 +/- 1.6 years). A significant elevation in BMI was detected in both groups during these periods (P < 0.0001 versus baseline, for both groups). The change in BMI (delta BMI) was similar in both groups (P < 0.9). We conclude that the increase in body weight caused by long-term (> 6 months) clozapine treatment is comparable to that obtained following long-term classical antipsychotic agents treatment.