A 6-week acute phase of an international 1-year double-blind study was conducted comparing three dose ranges of olanzapine (5 +/- 2.5 mg/day, 10 +/- 2.5 mg/day, and 15 +/- 2.5 mg/day) with a fixed dose of olanzapine (1.0 mg/day) and with a dose range of haloperidol (15 +/- 5 mg/day) in the treatment of 431 patients with schizophrenia. The purpose was to determine whether olanzapine demonstrated a dose-related ability to decrease overall psychopathology with minimal associated extrapyramidal symptoms in patients with schizophrenia. The high-dose olanzapine group showed statistically significantly greater improvement in overall psychopathology based on mean change in the CGI Severity score and statistically significantly greater improvement in positive psychotic symptoms based on mean change in both the BPRS positive score and the PANSS positive score compared with the 1.0-mg/day olanzapine group. Analyses indicated that an increasing dose-response curve was observed across the range of all olanzapine dose groups. Acute extrapyramidal syndromes were reported less frequently among all olanzapine groups compared with the haloperidol group. Endpoint mean change on both the Simpson-Angus Scale and the Barnes Akathisia Scale reflected improvement for all olanzapine treatment groups compared with worsening for the haloperidol group. Olanzapine was associated with weight gain but did not appear to have any clinically meaningful effect on vital signs. Although olanzapine was associated with some increase in prolactin concentrations, increases were transient, occurred less often, and were of lesser magnitude than those observed with haloperidol.