Olanzapine is an atypical antipsychotic effective in the treatment of schizophrenic patients. After a 2- to 9-day placebo lead-in, 79 inpatients with schizophrenia according to DSM-III-R criteria were placed on an olanzapine dosage of 10 mg/day or 1 mg/day for up to 6 weeks. Blood samples were obtained weekly during this period. Receiver operating characteristic curve analyses of Brief Psychiatric Rating Scale (BPRS) and Positive and Negative Syndrome Scale rating scale data suggested a minimum effective therapeutic concentration of 9 ng/mL. Using an intent-to treat analysis, 45% of the patients with olanzapine plasma concentrations > or = 9.3 ng/mL responded (> or = 20% decrease in BPRS), whereas only 13% of the patients with concentrations < 9.3 ng/mL responded. Use of olanzapine plasma concentrations of > 9 ng/mL as a predictor for treatment response in acutely ill schizophrenic patients is practicable because this therapeutic marker significantly increases the likelihood of a patient responding to olanzapine.