Patients with obstructive sleep apnea (OSA) have a high frequency of cardiovascular diseases and hypercoagulability is believed to be involved in the mechanism of those vascular events. We evaluated whether there is a dysfunction in the protein C anticoagulant pathway in patients with obstructive sleep apnea. Two hundred ninety-three patients were enrolled. To confirm the diagnosis of OSA, all-night polysomnography, including determination of SpO(2), was carried out. The apnea-hypopnea index (AHI) was used for judging the presence of sleep-breathing disorder. The plasma levels of the thrombin-antithrombin complex were higher in patients with AHI > 5 than in those without OSA, defined as AHI < 5. However, there was no statistically significant difference in the plasma level of either activated protein C/alpha-antitrypsin complex or soluble thrombomodulin between patients with AHI > 5 and those with AHI < 5. The results of this study showed for the first time that markers of the protein C anticoagulant pathway are not affected in patients with OSA and that the protein C pathway is probably not involved in the mechanism of hypercoagulability in subjects with sleep-disordered breathing.