The aim of this study was to characterize excessive daytime sleepiness (EDS) in a large cohort of Chinese patients with various severity of obstructive sleep apnea-hypopnea syndrome (OSAHS), and investigate its correlations with clinical/polysomnographic variables. A total of 1,035 consecutive Chinese patients with snoring (mean age +/- SD 45 +/- 15 years, BMI 26.6 +/- 4.3 kg/m(2)) were examined by overnight polysomnography, and subjective EDS was assessed using the Epworth Sleepiness Scale (ESS). The 1,035 patients were compared according to severity of sleep-disordered breathing: AHI < 5 (primary snoring group or normal overall AHI) (24.1%), AHI 5-20 (mild OSAHS, 21.7%), AHI > 20-40 (moderate OSAHS 16.5%), and AHI > 40 (severe OSAHS 37.7%). ESS score progressively increased as the severity of OSAHS aggravated among these patients. More severe OSAHS patients were characterized by EDS, nocturnal hypoxemia, and disruption of sleep structure. Progressive worsening of nocturnal hypoxemia was observed from mild to severe OSAHS patients with a strong correlation with ESS score. The stepwise multiple regression analysis performed to evaluate the correlations of individual clinical and polysomnographic variables with the ESS score revealed that the ESS score significantly correlated with the oxygen desaturation index (ODI), apnea-hypopnea index (AHI), and body mass index (BMI), and ODI was the strongest determinant of ESS score. EDS is correlated with the severity of OSAHS. More severe patients are characterized by higher ESS score, higher BMI, and progressive worsening of nocturnal hypoxemia. Nocturnal hypoxemia is a major determinant of EDS in Chinese OSAHS patients.