Background: The involvement of reactive species in pathophysiology of phenylketonuria (PKU) is well established. In previous studies it was verified that PKU patients (treated with a protein-restricted diet supplemented with a special formula not containing L-carnitine and selenium) presented high lipid and protein oxidative damage as well as reduction in antioxidants. Our goal in this study was to evaluate the effect of a supplementation with L-carnitine and selenium, two well-known antioxidant compounds, on oxidative stress in PKU patients. Methods: We investigated various oxidative stress parameters in blood of 18 treated PKU patients before and after 6 months of supplementation with a special formula containing L-carnitine and selenium.