Open access. Studies analyzing concentration-effect relationships in second-generation antipsychotics have reported contradictory results in chronic schizophrenia. No data are available for the early stages of the disease. The present study aims to evaluate the association between a single olanzapine plasma concentration, clinical response, and severity of adverse effects in first-episode psychosis (FEP); to test the utility of various plasma breakpoints as markers of early response to treatment; and to identify variables affecting olanzapine concentrations.