In patients with obstructive sleep apnea syndrome (OSAS), the blood coagulation system may contribute to an increased risk of cardiovascular events, which occur most frequently in the morning. Nasal continuous positive airway pressure (NCPAP) treatment can improve the mortality of patients with OSAS. We measured the plasma fibrinogen concentration, which is an independent risk factor for cardiovascular events, in the afternoon (3:30 P.M.) and the next morning upon awakening (8:30 A.M.) in 11 patients with OSAS (apnea and hypopnea index > 20) before and after NCPAP therapy. We also measured the hematocrit, the C-reactive protein, and the total plasma protein at the same time. The plasma fibrinogen and hematocrit levels in the morning (298 +/- 16 mg/dl and 48.5 +/- 1.5%, mean +/- SEM) were significantly higher than on the previous afternoon (275 +/- 14 mg/dl and 46.6 +/- 1.3%) (fibrinogen, p < 0.02; hematocrit, p < 0.005). The whole blood viscosity (WBV) at a shear rate of 208 inverse seconds, which can be predicted based on the hematocrit and total plasma protein, was also significantly higher in the morning (4.98 +/- 0.20/s) than in the afternoon (4.73 +/- 0.17/s) (p < 0.005). These increases in the plasma fibrinogen concentration and the WBV in the morning disappeared after NCPAP treatment. The attenuation of morning increases in the plasma fibrinogen concentration and WBV induced by NCPAP treatment may contribute to an overall improvement in the mortality from cardiovascular events in patients with OSAS.