Background: Obstructive sleep apnea syndrome (OSAS) and obesity hypoventilation syndrome (OHS) are two common syndromes that are associated with obesity. They frequently coexist. Patients with combined OSA-OHS have a higher risk of pulmonary hypertension as well as higher morbidity and mortality. The aim of the study is to estimate the prevalence of OHS in patients with OSA and to compare the demographic and clinical characteristics of patients with pure OSA and those with combined OSA-OHS. Methods: Ninety patients with OSAS were enrolled in this study. For all patients, the STOP BANG questionnaire and Epworth scale (ESS) were calculated, body mass index and neck circumference were measured, CBC, arterial blood gases, thyroid function tests and lipid profile were done. Adiponectin level was measured in all patients. PFT and echocardiography were done and a sleep study was done to confirm the diagnosis of OSA. Results: Patients were divided into two groups: group I included 74 patients with combined OSA-OHS and group II included 16 patients with pure OSA. BMI (P = 0.04), neck circumference (P = 0.001), and ESS (P = 0.03) were all statistically significantly higher in Group I patients. PASP was found to be elevated in both groups, with a statistically significant increase in group I patients. The AHI (P=0.04) and number of desaturations with Spo2 less than 90% (P= 0.016) were significantly higher in group I patients. Conclusion: OHS is a very common association with OSAS. The association of the two syndromes results in a greater increase in comorbidities.