To compare the semen quality score and the seminal oxidative stress reactive oxygen species (ROS) and total antioxidant capacity (ROS-TAC score) in men with idiopathic infertility with normal donors and to a known group of fertile and infertile men. Principal component analysis was applied to provide a standardized score in 36 men with idiopathic infertility and 19 controls attending our infertility clinic. A logistic regression analysis comparing the fertile and infertile men was used. Compared to controls, patients with idiopathic infertility had significantly lower sperm concentration, sperm motility and normal morphology (p < 0.05) and lower semen quality scores (83.0 +/- 14.5 vs. 100.0 +/- 10.0; p < 0.001). Compared to controls, the ROS levels were higher in the idiopathic infertility group (2.3 +/- 0.21 vs. 1.3 +/- 0.3; p = 0.006), whereas the TAC levels were lower in the idiopathic infertility (1,014.75 +/- 79.22 vs. 1,653 +/- 115.29; p = 0.001). Idiopathic infertility patients had lower ROS-TAC scores (32.8 +/- 14.2) than controls (50.0 +/- 10.0) (p < 0.001). 64% of men with idiopathic infertility will remain infertile during 1-year follow-up. Patients with idiopathic infertility have lower scores of semen quality and ROS-TAC. 2008 S. Karger AG, Basel