Background: As obesity increases, the frequency of obstructive sleep apnea and obesity hypoventilation syndrome increases also. However, obesity hypoventilation syndrome frequency is not known, as capnography and arterial blood gas analysis are not routinely performed in sleep laboratories. Aims: To investigate the frequency and predictors of obesity hypoventilation syndrome in obese subjects. Study Design: Retrospective clinical study Methods: Obese subjects who had arterial blood gas analysis admitted to the sleep laboratory and polysomnography were retrospectively analyzed. Subjects with restrictive (except obesity) and obstructive pulmonary pathologies were excluded. Demographics, Epworth- Sleepiness-Scale scores, polysomnographic data, arterial blood gas analysis, and spirometric measurements were recorded. Results: Of the 419 subjects, 45.1% had obesity hypoventilation syndrome. Apnea hypopnea index (p<0.001), oxygen desaturation index (p<0.001) and sleep time with SpO(2)< 90% (p<0.001) were statistically higher in subjects with obesity hypoventilation syndrome compared to subjects with eucapnic obstructive sleep apnea. The nocturnal mean SpO(2) (p<0.001) and lowest SpO2 (p<0.001) were also statistically lower in subjects with obesity hypoventilation syndrome. Logistic regression analysis showed that the lowest SpO(2), oxygen desaturation index, apnea hypopnea index and sleep time with SpO(2) < 90% were related factors for obesity hypoventilation syndrome. Conclusion: Obesity hypoventilation syndrome should be considered when oxygen desaturation index, apnea hypopnea index and sleep time with SpO(2) < 90% are high.