Objective To evaluate the prevalence and clinical characteristics of obstructive sleep apnea (OSA) in bariatric surgery population.Methods Consecutive patients undergoing preoperative evaluation for bariatric surgery and referred for sleep monitoring in Peking Union Medical College Hospital from January 2009 to December 2015 were retrospectively analyzed. Age,gender,symptoms of snoring,fatigue,apnea and somnolence,apnea hyponea index (AHI),arterial blood gas,pulmonary function,sleep respiratory monitoring,positive airway therapy of OSA,and postoperative complications were recorded. The clinical characteristics of OSA groups at different severity were compared using AHI≥5 events/hour and AHI≥15 events/hour as cut-off values. Correlation analysis was applied to identify the clinical factors associated with AHI.Results Of 42 patients with complete sleeping monitoring data before bariatric surgery,30(71.4%) were diagnosed as OSA,14 (33.3%) were moderate or severe and 11 (26.2%) were severe. Also,OSA was not detected before the bariatric surgery in 25 cases (83.3%). Compared with patients with AHI<5 events/hour,patients with AHI≥5 events/hour had significantly older age (t=2.869,P=0.007),higher proportion of observed apnea (P=0.035),higher AHI (z=-4.592,P=0.000),higher proportion of night pulse oxygen saturation(SpO2) below 90% (z=-2.746,P=0.006),lower mean SpO2 (t=-2.071,P=0.046) and lower lowest SpO2 (t=-3.914,P=0.000). Compared with AHI<15 events/hour group,the AHI≥15 events/hour group had significantly higher BMI (t=2.281,P=0.043),male ratio (P=0.005),incidence of hypertension (P=0.011),proportion of observed apnea (P=0.001),percentage of smoking history (P=0.017),partial pressure of carbon dioxide(PaCO2)(t=3.478,P=0.002),AHI (z=-4.592,P=0.000),and proportion of night SpO2 below 90% (z=-4.530,P=0.000); in addition,the forced expiratory volume in one second(FEV1)% predicted (t=-3.377,P=0.002),forced vital capacity(FVC)% predicted (t=-2.342,P=0.026),night time mean SpO2 (t=-3.392,P=0.007),lowest SpO2 (t=-5.535,P=0.000) were significantly decreased. Correlation analysis showed that,in populations with normal PaCO2 (n=36),AHI was positively correlated with age (r=0.450,P=0.006) and BMI (r=0.384,P=0.021) and negatively correlated with FEV1% predicted (r=-0.457,P=0.008) and FVC% predicted (r=-0.432,P=0.013). Partial correlation analysis showed that,after age and BMI were adjusted,AHI was not correlated with FEV1% predicted(r=-0.287,P=0.125)and FVC%predicted(r=-0.241,P=0.200).Conclusion The incidence and underdiagnosis rate of OSA are high in bariatric surgery population. OSA should be routinely screened in bariatric population to reduce the postoperative complication.