Background The nocturnal nondipping and elevated morning blood pressure (BP) in patients with obstructive sleep apnea syndrome (OSAS) have not yet been well investigated in Chinese patients. This study aimed to describe the BP profile, and to elucidate the relationships between daytime BP and nighttime BP, and between evening BP and morning BP in patients with OSAS. Methods Twenty teaching hospital sleep centers in China were organized by the Chinese Medical Association to participate in this study and 2297 patients were recruited between January 2004 and April 2006. BP assessments were made at four time points (daytime, evening, nighttime and morning) and polysomnography (PSG) was performed and subjects were classified into four groups by their apnea-hypopnea index (AHI): control, n=213 with AHI <5; mild, n=420 with AHI >= 5 and <15; moderate, n=460 with AHI >= 15 and <30; and severe, n=1204 with AHI SPSS 11.5 software package was used for statistical analysis and figure drawing. Results All the average daytime, nighttime, evening and morning BPs were positively correlated with AHI and negatively correlated with nadir nocturnal oxygen saturation. The ratios of nighttime/daytime and morning/evening average BP were positively correlated with AHI. The ratio of nighttime/daytime systolic BP became a "reversed BP dipping" pattern until the classification reached severe, while the ratio of nighttime/daytime diastolic BP became reversed at moderate. Similarly, the ratio of morning/evening diastolic BP becomes reversed even at mild. Conclusions OSAS may result in higher BP levels at all four time points. The ratios of nighttime/daytime and morning/evening BP increase with increased AHI. The increasing of diastolic BP, which is inclined to rise more quickly, is not parallel with increasing systolic BP. Chin Med J 2012;125(10):1740-1746