OBJECTIVE: Blood biomarkers for diseases have recently become a topic of great interest. Many studies of Autism Spectrum Disorder (ASD) have been made to date looking for biomarkers in peripheral tissues, but no specific biomarker has yet been found. The aim of this study was to examine oxidative stress parameters including malondialdehyde (MDA) levels, superoxide dismutase (SOD), and catalase (CAT) activity and to determine both their sensitivity and specificity as biomarkers associated with oxidative stress in ASD. METHODS: This study measured the plasma MDA levels, SOD, and CAT activities in erythrocyte in 52 patients with ASD (aged 3–6 years) and in 48 age- and gender-matched healthy controls. ASD severity was rated using the Childhood Autism Rating Scale (CARS). RESULTS: MDA levels, SOD, and CAT activity were significantly higher in patients with ASD in comparison to the controls (p