This pooled analysis of four 6-week, randomized, open-label, parallel trials of patients with schizophrenia (Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition) compared the efficacy and tolerability of olanzapine (5-20 mg/d) with those of chlorpromazine (200-800 mg/d). Of the 123 patients randomly allocated to olanzapine (n = 83) or chlorpromazine (n = 40), 109 completed the study (olanzapine, n = 77; chlorpromazine, n = 32). Olanzapine-treated patients showed significantly greater baseline-to-end point mean improvements in the primary efficacy measure, Brief Psychiatric Rating Scale total, compared with chlorpromazine-treated patients (least-squares means: olanzapine, -21.1; chlorpromazine, -10.4; P < 0.001). Response rate was significantly higher in the olanzapine group (66.3% vs. 32.4%; P = 0.001). Baseline-to-maximum changes in the UKU scores for akathisia were significantly different between the groups (P = 0.018). A decrease (improvement) in these scores was observed in 6/74 (8.1%) of olanzapine- and 1/36 (2.8%) chlorpromazine-treated patients. Weight gain was the only common (> or =10%) adverse event that occurred more frequently, although not significantly differently, in the olanzapine group (27.7%) than the chlorpromazine group (12.5%), whereas postural hypotension was the only common adverse event whose occurrence was significantly different between the groups (olanzapine, 0.0%; chlorpromazine, 10.0%; P = 0.010). Both the incidence of > or =7% weight gain from baseline (olanzapine, 26.3%; chlorpromazine, 24.3%) and baseline-to-end point changes in weight (mean +/- SD, kg: olanzapine, 3.41 +/- 3.14; chlorpromazine, 2.81 +/- 2.65) were not significantly different between the treatment groups. Baseline-to- end point changes in nonfasting glucose differed significantly between the groups (mean +/- SD, mmol/L: olanzapine, 0.09 +/- 1.11; chlorpromazine, 0.72 +/- 2.04; P = 0.042). This analysis suggests that, compared with chlorpromazine, olanzapine may be more efficacious and have a more favorable tolerability profile in treating patients with schizophrenia.