OBJECTIVES: Serum adiponectin levels have been found to be lower in patients with obesity, hypertension, and coronary artery diseases. In this study, we aimed to evaluate serum adiponectin levels in patients with obstructive sleep apnea syndrome (OSAS) and to correlate these levels with the severity of OSAS. STUDY DESIGN: In 62 OSAS patients (39 males, 23 females) and 32 controls (23 males, 9 females) determined by polisomnography, serum adiponectin levels were analyzed by the ELISA method. Patients were classified as having either mild (apnea hypopnea index, AHI: 5-14), moderate (AHI: 15-29) or severe (AHI ≥30) OSAS, and controls were defined as AHI <5. Plasma fasting glucose, total cholesterol (TC), triglyceride (TG), and high (HDL-C) and low (LDL-C) density lipoprotein cholesterols were analyzed, and the results were compared between the groups. RESULTS: There was no significant difference in mean age (51.6±10.7 years for patients, 48.3±10.8 years for controls) or body mass index (32.9±6.0 kg/m2 for patients, 31.3±5.6 kg/m2 for controls, p>0.05) in our study population. There was no significant difference in the number of hypertensive, diabetics, or smokers between the patients and controls. While serum TC, TG, and HDL cholesterol levels were not significantly different between two groups, the serum adiponectin levels of patients (3.0±3.4 µg/dl) were significantly lower than those of the controls (5.2±5.2 µg/dl, p=0.01). While serum adiponectin levels showed a significantly negative correlation with AHI (r=-0.221, p=0.03), there was a significantly positive correlation with minimum and mean oxygen saturations (r=0.213, p=0.04 and r=0.205, p=0.05). CONCLUSION: Serum adiponectin levels were significantly lower in patients with OSAS, especially for those in the severe OSAS group. Serum adiponectin levels are related to the severity of OSAS and arterial oxygen saturation.