BACKGROUND: Chronic intermittent hypoxia, platelet activation and inflammation all play roles in the pathogenesis of obstructive sleep apnea syndrome (OSAS), which may increase the risk of cardiovascular disease (CVD). The aim of this study was to evaluate the relationship of the platelet-to-lymphocyte ratio (PLR) as a new biomarker showing systemic inflammation and platelet distribution width (PDW) as an indicator of platelet activation to the severity of OSAS. METHODS: A total of 290 patients suspected with OSAS who underwent a full night of polysomnography were included. The patients were placed into 4 separate groups according to their apnea-hypopnea index (AHI) scores; the control group (AHI <5), mild OSAS group (AHI 5-15), moderate OSAS group (AHI 16-30), and severe OSAS group (AHI >30). CVD risk was defined by the presence of hypertension, diabetes mellitus, current smoking, and dyslipidemia. RESULTS: Higher AHI groups were significantly correlated with increasing age, body mass index, systolic blood pressure and male sex. PLR and PDW were also significantly associated with AHI (r = 0.417 for PLR and r = 0.227 for PDW, all p-values < 0.001) and the Epworth sleepiness scale (r = 0.160 for PLR and r = 0.189 for PDW, all p-values <0.05). Multivariate regression analysis revealed that AHI < or =9.2 (adjusted odds ratios [OR] 5.03, 95 % confidential interval (CI) = 1.67-15.2, p = 0.004) and PLR > or =159 (adjusted OR 2.81, 95 % CI = 1.34-5.91, p = 0.006) were independently associated with the presence of hypertension. CONCLUSION: PLR and PDW are associated with OSAS severity. PLR may also be useful as a systemic biomarker for the concurrent hypertension in OSAS patients.