Olanzapine is an atypical antipsychotic drug used in the treatment of schizophrenia. Controversial results have been obtained measuring different serum antioxidant enzymes and serum malondialdehyde (MDA) in schizophrenic patients treated with olanzapine. The aim of this study is to find the effect of olanzapine on total antioxidant status (TAS) and lipid peroxidation in schizophrenic patients. Thirty schizophrenic patients were treated orally with olanzapine (10–20 mg/day) for 2 months. Thirty healthy subjects were also included as a control group. Blood samples were taken from patients before and after olanzapine therapy, and analyzed for serum TAS and MDA. In schizophrenic patients, mean values of pretreatment serum TAS were significantly less (difference = 37.4% of control) than the control value, whereas serum MDA levels were significantly higher (difference = 176% of control) than the control values . Olanzapine treatment for 2 months significantly increased serum TAS levels (37.8%) and reduced serum MDA levels (22.2%) in comparison to respective pretreatment values. In conclusion, the data suggest that olanzapine therapy for 2 months at least partially ameliorates adverse effects on the antioxidant defense mechanism in schizophrenia.