The role of carnitine supplementation in idiopathic asthenozoospermia was investigated. Thirty asthenozoospermic patients were divided into two groups according to phospholipid hydroperoxide glutathione peroxidase (PHGPx) levels and received placebo for 3 months, then oral L-carnitine (2 g/day) for 3 months. Semen samples were collected at baseline, after placebo, after carnitine administration and again after 3 months with no drugs. When asthenozoospermic subjects were divided into two groups on the basis of PHGPx levels, an improvement of mean sperm motility was observed only in the group of patients with normal PHGPx levels. PHGPx has an important role in male infertility and carnitine treatment might improve sperm motility in the presence of normal mitochondrial function.