In this study, it was aimed to determine the effects of enteritis, pneumonia and omphalitis on oxidative/antioxidant balance in the calves. In total, 27 calves with neonatal disease and 10 healthy calves (control) were used. The sick calves were divided into 3 groups according to localization of infection, omphalitis (n = 10), pneumonia (n = 8) and enteritis (n = 9). Blood samples of the calves were taken from jugular vein for analysis. Blood in tubes without anticoagulant was centrifuged at 5000 rpm for 5 min following clotting, and the sera were removed and stored at -80°C until analyzed. Serum malondialdehyde and nitric oxide levels of all groups were significantly higher, while glutathione, glutathione peroxidase, catalase and superoxide dismutase levels were significantly lower compared to the control group. As a conclusion, diarrhea, pneumonia and omphalitis are caused by oxidative damage in the calves, and antioxidant treatment may be beneficial in the treatment of these diseases.