OBJECTIVE: Therapeutic drug monitoring (TDM) indicators have been suggested to predict overall outcome responses to olanzapine (OLZ) treatments in terms of efficacy and metabolic syndrome. This study aimed to investigate whether paraoxonase-1 (PON-1) activity can be used to predict schizophrenia patient outcomes. METHODS: Schizophrenic patients ( RESULTS: PON-1 activity and OLZ/D were significantly correlated in 50 patients (correlation coefficient, CONCLUSIONS: PON-1 activity can be used as an alternative tool for monitoring TDM through the measurement of OLZ together with its metabolite, DMO, to identify patients who have higher activity. Those who show an optimal response or who have lower activity might have greater cardiometabolic risk under long-term olanzapine treatment. Longitudinal monitoring is warranted to confirm such observations.