The most common cause of male infertility is idiopathic. Standard investigations reveal no abnormality in such cases. The aim of the study was to investigate the levels of sperm DNA damage and seminal oxidative stress and their relationships with idiopathic infertility. The study included 30 normozoospermic infertile men seeking infertility treatment and 20 fertile donors. Semen analysis was performed according to the World Health Organization guidelines. Sperm DNA damage was assessed by alkaline single cell gel electrophoresis (comet assay) after preparation with two-step discontinuous Percoll gradient. Seminal oxidative stress was measured by a novel automated method. DNA damage score, total antioxidant status (TAS), total oxidant status (TOS) and oxidative stress index (OSI) were not different in idiopathic infertile men compared with controls. No correlations were also found between DNA damage score and TAS, TOS levels and OSI in idiopathic infertile group. We did not find any relationship between sperm DNA damage and oxidative stress in normozoospermic infertile men. We think that the pathophysiology of idiopathic infertility cannot be explained by sperm DNA damage or seminal oxidative stress. © Springer-Verlag 2006.