Background: Obstructive sleep apnea syndrome (OSAS) is a common sleep disorder characterized by recurrent upper airway collapse during sleep. Recently, some hematological parameters as red cell distribution width (RDW), mean platelet volume (MPV), and platelet distribution width (PDW) have been emerged as inflammatory biomarkers. Limited, controversial data is available on the association between these hematological parameters and the severity of OSAS. Therefore, we aimed to investigate the levels of these parameters in patients with OSAS and its correlation with the severity of OSAS. Methods: The clinical data, polysomnography and laboratory results of complete blood pictures of 116 patients with OSAS were retrospectively collected and statistically analyzed. Results: Obstructive sleep apnea syndrome was associated with increased levels of hematocrit, RDW, MPV, PDW, and platelets count. RDW is positively correlated with Apnea-hypopnea index, oxygen desaturation index, Epworth sleepiness scale, and negatively correlated with minimum oxygen saturation and rapid eye movement sleep. Conclusion: RDW may be a marker for the severity of OSAS. As RDW is included in a complete blood count, it could provide an easy, inexpensive tool for triaging OSAS patients for polysomnography evaluation.