Oxidative stress has been proposed as being important in the pathophysiology of autism spectrum disorders (ASD), and heightened levels of oxidative stress has found in children with ASD. Our aim was to investigate, whether this change is temporary or persist into adulthood. We included 89 adult patients with ASD and sex and age matched controls. Plasma levels of antioxidants superoxide dismutase 1 (SOD1) and superoxide dismutase 2 (SOD2) and pro-oxidant xanthine oxidase (XO) were measured. Individuals with ASD had higher levels of SOD1, which furthermore correlated with autism severity as measured by autism quotient-score. We found no difference regarding SOD2 and XO between ASD group and controls. However, SOD1 and SOD2 were elevated in males compared to females.