The aim of the present study was to investigate serum paraoxonase/ arylesterase activities and oxidation/ oxidizability of apolipoprotein B- containing lipoproteins and several coronary artery disease risk factors, including homocysteine, high sensitive C- reactive protein, tumour necrosis factor- alpha, leptin and adiponectin in patients with schizophrenia. Oxidation of lipoproteins plays an important role in atherogenesis, and the enzyme paraoxonase has been shown to prevent lipoprotein oxidation. Furthermore, low paraoxonase activity has been suggested to predict coronary artery disease. Forty patients who fully met the fourth Diagnostic and Statistical Manual of Mental Disorders criteria for schizophrenia and 35 healthy control subjects were included in the study. Serum paraoxonase/ arylesterase activities were determined spectrophotometrically. Malondialdehyde levels of apolipoprotein B- containing lipoproteins were determined before and after incubation with copper- sulphate, which yielded basal- and Delta- malondialdehyde values, respectively. Homocysteine and highly sensitive C- reactive protein levels were determined using a fluorescence- polarization immunoassay and immunonephelometry, respectively. Leptin and adiponectin levels were measured with radioimmunoassay and tumour necrosis factor-alpha was determined by enzyme linked immunosorbent assay. Serum paraoxonase and arylesterase activities were significantly lower and Delta- malondialdehyde levels were significantly higher in the schizophrenia group compared with the control group. However, there were not any significant differences in other parameters of the study between the study groups. There was a significant increase in body mass index and serum triglyceride and very low density lipoprotein cholesterol levels in the schizophrenic group after 6 weeks of treatment. These parameters were significantly increased in patients treated with atypical antipsychotics but not in patients treated with typic or long acting antipsychotics. The results of the present study suggest that patients with schizophrenia might have increased risk for coronary artery disease related to reduced serum paraoxonase activity and increased oxidizability of apolipoprotein B- containing lipoproteins.