Platelet activation and inflammation are the related mechanisms of pathogenesis in obstructive sleep apnea syndrome (OSAS). Mean platelet volume (MPV) and platelet distribution width (PDW) are the markers of platelet activation. C-reactive protein (CRP) and red cell distribution width (RDW) in relation to the inflammation in OSAS might be increased. We aimed to investigate the correlation of the MPV, PDW, and RDW levels with the severity of OSAS. We included 98 patients with suspected OSAS, evaluated by polysomnography. According to their apnea-hypopnea index (AHI), patients were divided into group A (n = 20; 20.4%) with AHI below 5/h; group B (n = 15; 15.3%) with AHI 5-14.9/h; group C (n = 26; 26.5%) with AHI 15-29.9/h; and group D (n = 37; 37.8%) with AHI ≥30/h. Hemoglobin, MPV, PDW, and RDW were measured using an automated blood cell counter. BMI and age showed a statistically significant and gradual increase in AHI groups. Hemoglobin, platelet, CRP, MPV, and RDW values did not differ between AHI groups. PDW was significantly higher in group D (mean value 14.4 ± 1.8) than in group A (13.2 ± 0.5) (P < 0.001). Epworth sleepiness scale was significantly higher in group D (9.9 ± 5.5) than in group A (5.6 ± 3.5), B (9.2 ± 3.6) and C (6.6 ± 3.8) (P = 0.005). When the four groups were compared, group D had the lowest minimum SpO2 value [group A (89.4 ± 3.0), B (86.7 ± 4.2), C (81.2 ± 6.4), and D (68.2 ± 13.0)]. There was a statistically significant correlation between AHI and age (r = 0.35, P < 0.001), BMI (r = 0.31, P = 0.003), PDW (r = 0.28, P = 0.006), and Epworth sleepiness scale (r = 0.29, P = 0.007). However, AHI was not correlated with CRP, MPV, and RDW. PDW is higher in severe OSAS and is correlated with different parameters of breathing function during sleep. The severity of OSAS was not correlated with CRP, MPV, and RDW. These findings show an evidence for platelet activation in OSAS and suggest that PDW might be related markers of OSAS severity.