Background/aim: This study aimed to evaluate whether NREM-predominant obstructive sleep apnea syndrome (OSAS) patients (NREM AHI < REM AHI) have distinct clinical and polysomnographic features compared to REM-predominant OSAS patients (REM AHI > NREM AHI). Materials and methods: A total of 342 patients (93 females and 249 males) who were admitted to the Sleep Disorders Unit at the Gazi University Faculty of Medicine and underwent polysomnography between January 2011 and April 2016 were retrospectively reviewed. Patient data, symptoms related to OSAS, Epworth Sleepiness Scale (ESS) scores, and polysomnographic findings were recorded. The patients were divided into two groups according to the apnea-hypopnea index (AHI) as patients with NREM-predominant OSAS and patients with REM-predominant OSAS. Results: The total AHI in the NREM-predominant group was significantly higher than in the REM-predominant group (P < 0.001). The patients with severe OSAS constituted the majority in both groups, and the rate of patients with severe OSAS was significantly higher in the NREM-predominant group than in the REM-predominant group (P < 0.001). Arousal index and sleep time spent under 90% SaO2 was higher in the NREM-predominant group (P = 0.005, P = 0.001), whereas nocturnal mean and minimum O2 saturation values were lower in the NREM-predominant group compared to patients with REM-predominant OSAS (P < 0.001, P = 0.013). In evaluating systemic disorders, the prevalence of coronary artery disease was significantly higher in the NREM-predominant OSAS group (P < 0.001). Conclusion: Our results showed that patients with NREM-predominant OSAS had a more severe course than patients with REMpredominant OSAS. However, we found no significant difference in sleep-specific symptoms, suggesting that the two groups represented distinct entities.