Obstructive sleep apnea (OSA) is characterized by recurrent upper airway obstruction resulting in apneas, hypopneas, oxygen desaturation, and arousal from sleep. Obesity is one of the most significant risk factors for OSA. Although continuous positive airway pressure is considered the first choice of treatment of OSA, weight reduction is another important measure in obese OSA patients. Conversely, a couple of studies showed that OSA itself might lead to increase weight gain by the reduction in sleep and/or the daytime sleepiness. And obesity hypoventilation syndrome (OHS) is defined as chronic daytime hypercapnia in combination with obesity and OSA. OHS patients are more likely to suffer from congestive heart failure, pulmonary hypertension and have worse prognosis than obese eucapnic OSA patients.