<p>Background: The newest NOX isoform, NOX5, has been found in mammalian spermatozoa. Many physiological and pathological situations in spermatozoa are mediated by reactive oxygen species (ROS). NOX5 is the main source of ROS in spermatozoa. Our purpose was to investigate the changes in NOX5 expression and the effect of NOX5 expression on sperm motility, chromatin integrity, and oxidative status in oligoasthenozoospermic compared to normozoospermic men.<br /> Methods: Semen samples were collected from 30 normozoospermic (NS) and 30 oligoasthenozoospermic (OAS) men. NOX5 protein expression in sperm samples was evaluated by immunohistochemistry and western blot. Oxidative stress status was evaluated by total antioxidant capacity (TAC), total oxidant capacity (TOC), and oxidative stress index (OSI) parameters. Chromatin integrity in spermatozoa was evaluated by toluidine blue staining.<br /> Results: NOX5 expression levels were significantly higher in OAS group than in NS group (p&lt;0.001). In addition, chromatin integrity was significantly higher in the OAS group in comparison to NS group (p&lt;0.001). TAC levels were higher in the NS group, but OSI and TOC levels were significantly higher in OAS group (p&lt;0.001). It was found that NOX5 protein expression was positively correlated with oxidative stress and chromatin integrity and negatively correlated with motility (p&lt;0.01).<br /> Conclusion: These results suggest that overexpression of NOX5 may be the source of excessive ROS production and oxidative stress injuries in oligoasthenozoospermic men. Considering that NOX5 expression is positively correlated with oxidative stress and chromatin integrity but negatively correlated with motility, it can be considered a biomarker to be used in assisted reproductive procedures.</p>