The aim of this study was to investigate the relationship between the subjective Epworth Sleepiness Scale (ESS) and objective polysomnographic parameters that was compared in dichotomized groups of obstructive sleep apnea (OSA) patients with and without excessive daytime sleepiness (EDS). A retrospective review of the 96 consecutive snoring patients suspected of OSA due to upper airway problems was performed. ESS score was assessed and EDS was considered present when the ESS score was >10. Polysomnography was performed for each patient. Comparisons of polysomnographic parameters between both groups of patients were performed using Mann-Whitney U-tests. The Pearson's correlation coefficient was calculated. EDS patients showed significantly greater body mass index (BMI), AHI and ODI, higher percentage of snoring time, a longer time length of SpO(2) < 90% and lower minimum SpO(2) compared to no EDS patients. There was no statistical difference in other sleep parameters. A positive correlation between the ESS score and BMI, percentage of snoring time, minimum SpO(2) and time length of SpO(2) < 90% was identified. Despite many controversies, our conclusion is that apnea/hypoxemia has considerable coherence to account for the EDS measured by ESS in OSA patients.