['C-reactive protein (CRP) and interleukin (IL)-6 are important risk factors for atherosclerosis and coronary heart disease. In the present study, we examined serum levels of CRP and IL-6, IL-6 production by monocytes, and the effect of nasal continuous positive airway pressure (nCPAP) in patients with obstructive sleep apnea syndrome (OSAS).', 'After polysomnography, venous blood was collected at 5 AM from 30 patients with OSAS and 14 obese control subjects. Serum levels of CRP and IL-6 and spontaneous production of IL-6 by monocytes were investigated. In addition, the effects of 1 month of nCPAP were studied in patients with moderate to severe OSAS. Levels of CRP and IL-6 were significantly higher in patients with OSAS than in obese control subjects (CRP P<0.001, IL-6 P<0.05). IL-6 production by monocytes was also higher in patients with OSAS than in obese control subjects (P<0.01). In patients with OSAS, the primary factors influencing levels of CRP were severity of OSAS and body mass index and those influencing levels of IL-6 were body mass index and nocturnal hypoxia. nCPAP significantly decreased levels of both CRP (P<0.0001) and IL-6 (P<0.001) and spontaneous IL-6 production by monocytes (P<0.01).'] Are elevated levels of C-reactive protein and interleukin-6 in patients with obstructive sleep apnea syndrome decreased by nasal continuous positive airway pressure?