Oxidative stress has been implicated in the pathogenesis of male infertility. Paraoxonase-1 (PON-1) is a high-density lipoprotein-associated antioxidant enzyme that prevents oxidative modification of low-density lipoprotein. Our aims in the study were to investigate 1) seminal PON-1 activity in subfertile men and 2) whether seminal PON-1 activity had any relationship to semen parameters. The study included 28 men with idiopathic subfertility, 32 subfertile men with abnormal semen parameters, and 30 fertile male volunteers. Seminal PON-1 activity was measured spectrophotometrically. Seminal total antioxidant status (TAS) and total oxidant status (TOS) were determined by using colorimetric methods. Oxidative stress index (OSI) was calculated as ([TOS/TAS] x 100). TOS and OSI were significantly higher and PON-1 activity and TAS were significantly lower in subfertile men with abnormal semen parameters than in men with idiopathic subfertility and fertile donors. PON-1 activity was also strongly correlated with sperm concentration (r = .68, P < .0001), motility (r = .58, P < .0001), and morphology (r = .62, P < .0001) in the overall group. The receiver operating characteristic curve analysis revealed a high diagnostic value for PON-1 activity with respect to male-factor subfertility, with an area under curve of .95 (95% confidence interval = 0.89-1.01), sensitivity = 97%, and specificity = 88%. Men with abnormal semen parameters have decreased levels of PON-1 activity in their seminal plasma. This may play an important role in the pathogenesis of male-factor subfertility.