Objective: Obstructive sleep apnea syndrome (OSAS) is a syndrome defined frequently by decrease in arterial oxygen saturation and repeated upper airway obstruction episodes during sleep. The most important complications of OSAS patients belong to cardiovascular system. Systemic arterial hypertension (43-60%), pulmonary hypertension (20-30%), coronary artery disease (20-30%) and congestive heart failure (5-10%) are among OSAS associated cardiovascular disease spectrum. In this study, we investigated the frequency of hypertension (HT), which is the most common cardiovascular disease seen in patients admitted with OSAS suspicion. Methods: Overall 263 consecutive patients with possible OSAS were included in this cross-sectional study. Diagnostic polysomnography (PSG) was performed in all patients with 44-channel polysomnograph (Compumedics E series, Australia) one whole night. Sleep stages and respiratory events were manually scored. Patients with apnea hypopnea index (AHI) <5 were diagnosed as OSAS negative, patients with AHI between 5-14 were diagnosed as mild OSAS, patients with AHI between 15-29 were diagnosed as moderate OSAS, patients with AHI ≥ 30 were diagnosed as severe OSAS. Relation of OSAS and presence of HT was statistically analyzed with Chi-square test. Results: According to PSG records of the patients who were assessed with OSAS suspicion, 54 (20.5%) patients were assessed as OSAS negative, 26(9.9%) - mild OSAS, 45 (17.1%) - moderate OSAS, and 138 (52.5%) were diagnosed as severe OSAS. Hypertension was present in 4 (7.4%) of 54 OSAS negative patients and 56 (36.8%) of 209 patients with OSAS, the difference was significant (p=0.001). When we assessed mild OSAS patients and moderate-severe OSAS patients in terms of HT frequency, HT was present in a lower rate in mild OSAS patients as compared with patients with moderate-severe OSAS (3/26 versus 53/183; p=0.044). When we compared OSAS negative patients with moderate-severe OSAS patients, HT was less frequently found in OSAS negative patients (4/54 versus 53/183, p=0.001). Conclusion: It is determined that hypertension was more frequently seen in patients with OSAS than in patients without OSAS and that HT frequency increased in parallel to the severity of OSAS.