Objective: To observe the risk factors of vascular thrombotic diseases in patients with obstructive sleep apnea hypopnea syndrome (OSAHS). Methods: From February of 2003 to April of 2005 24 patients with moderate and severe OSAHS (OSAHS group) and 19 healthy adults (control group) were recruited. Their blood samples were drawn at 6:00 and 16:00 respectively for testing hemocrit, platelet aggregation (PAG), whole blood viscosity (WBV), prothrombin time (PT), activated partial thromboplastin time (APTT), plasma fibrinogen (Fng) and endothelin (ET). Results: There was a significantly higher hemocrit, WBV, Fng and ET as well as a significant shortening of PT and APTT at 6:00 than that at 16:00 in OSAHS group. However, there was no significant difference in all testing items between 6:00 and 16:00 in control group. The hemocrit, WBV, PAG, plasma Fng and ET were significantly higher, and PT and APTT were obviously shorter at 6:00 in OSAHS group than those at 6:00 in control group. A higher hemocrit, PAG, plasma Fng and ET, a longer PT and APTT were observed at 16:00 in OSAHS group, compared with those at 16:00 in control group. Conclusions: In OSAHS patients there were striking risk factors of thrombosis, which is more remarkable in the early morning than in the afternoon.