Olanzapine is a new antipsychotic agent with serotonin/dopamine antagonism action. Efficacy in treating overall psychopathology in acute schizophrenia as measured by the BPRS0-6 total score was demonstrated at 10 mg/day versus placebo; at doses in a 5-20 mg/day range, olanzapine was comparable or superior to haloperidol. Superior efficacy for negative and depressive symptoms was shown in comparison to haloperidol. Olanzapine has a favorable acute and tardive extrapyramidal symptom profile relative to haloperidol and caused substantially less elevation of serum prolactin. Dose-related weight gain and asymptomatic mild transaminase elevations occurred in olanzapine-treated patients.