Obstructive sleep apnea (OSA) is associated with increased cardiovascular morbidity and mortality. Free radicals and adhesion molecules were implicated in the pathogenesis of atherosclerosis leading to cardiovascular disorders. Therefore, we investigated the link between CD15, CD11c, CD11b, and CD64 expression on leukocytes and their ability to generate reactive oxygen species (ROS) in patients with OSA and control volunteers. We also studied the effects of hypoxia in vitro on monocytes from control subjects and the ability of monocytes from both groups to adhere to human endothelial cells in culture. The effect of nasal continuous positive airway pressure (nCPAP) treatment was studied as well. We found that OSA was associated with increased expression of adhesion molecules CD15 and CD11c on monocytes, increased adherence of monocytes in culture to human endothelial cells, increased intracellular ROS production in some monocyte and granulocyte subpopulations, and upregulation of CD15 expression due to hypoxia in vitro in monocytes of control subjects. Furthermore, nCPAP treatment was associated with downregulation of CD15 and CD11c monocyte expression and decreased basal ROS production in CD11c+ monocytes. Monocyte adherence to endothelial cells decreased as well. Our findings provide one of the possible mechanisms for explaining the high rate of cardiovascular morbidity in patients with sleep apnea.