Objectives: Comparing the levels of total antioxidant capacity (TAC) and activity of antioxidant enzymes superoxide dismutase (SOD), and catalase (CAT) in PKU compared to the controls. Material and methods: We studied 42 PKU under dietary treatment and compared with 30 controls. TAC levels and SOD activity were determined by enzymatic spectrometry and CAT activity by spectrophotometry. Results: The activity of CAT and SOD is decreased significantly in PKU (CAT: p=0.02, SOD: p=0.001). We found no significant differences in TAC levels between PKU and controls. Conclusion: The antioxidant enzymes CAT, SOD are considered the main enzymatic defences against free radical production, and are decreased significantly in PKU in our area, which reinforces the hypothesis that oxidative stress is increased in PKU, and that pathogenic processes also occur in patients under protein dietary. Background: Phenylketonuria (PKU) is caused by deficiency of the phenylalanine hydroxylase, leading to accumulation of phenylalanine. Clinical features of PKU include mental retardation, microcephaly, and seizures. Mechanisms of brain damage still not clear, but increased oxidative stress is associated.