Objective To explore the contribution of obstructive sleep apnea hypopnea syndrome(OSAHS) in the variations of blood pressure in the evening to morning and possible mechanisms.Methods In Sleep and Breathing Disorders Centre，from September 2003 to September 2007，adult patients whose Epworth sleeping scoregt;9 were undergone polysomnography(PSG) and divided into 4 groups according to apnea hyponea index(AHI).The levels of blood pressure were monitored and compared between evening and morning.Correlations between PSG indexes and variations of the systolic blood pressure(SBP) and diastolic blood pressure(DBP) were analyzed in OSAHS patients.Results 1 528 patients were enrolled in this study.There was no significant difference between the evening and morning blood pressure in the non-OSAHS group(AHIlt;5，n=172)，whereas DBP rised about 1.73 mm Hg in the mild OSAHS group(AHI≤20，n=435)，SBP and DBP rised about 3.52 and 3.71 mm Hg respectively in the moderate OSAHS group(AHI≤40，n=307)，and SBP and DBP rised about 3.72 and 4.22 mm Hg respectively in the severe OSAHS group(AHIgt;40，n=614).The variation of SBP during the night correlated positively with the arousal index in the mild OSAHS group（r=0.25，Plt;0.05），but with the body mass index (BMI) in the moderate OSAHS group（r=0.25，Plt;0.05）.In the severe OSAHS group，the variation of SBP during the night correlated positively with BMI and the longest apnea time (LA)（r=0.26，0.25，both Plt;0.05），the variation of DBP during the night correlated positively with AHI and mean apnea duration(MA)（r=0.22，0.17，both Plt;0.05），and the variation of mean arterial pressure during the night correlated positively with AHI and MA（r=0.25，0.20，both Plt;0.05）.Conclusion OSAHS may induce mild rises of the blood pressure at night.The relevant factors that influence the blood pressure are different in different severity of the OSAHS.