Background: Leptin is a hormone with well-investigated functions concerning body composition, energy homeostasis and feeding behavior in humans. The obstructive sleep apnea syndrome (OSAS) is strongly associated with obesity, which is known to be closely associated with hyperleptinemia. More recently, ghrelin, a hormone that also influences appetite and energy homeostasis, has been discovered. Objectives: The aim of this study was to investigate serum leptin and ghrelin levels in obese patients with OSAS in comparison with equally obese controls without OSAS. Methods: Thirty untreated obese patients with moderate-severe OSAS (apnea-hypopnea index: AHI >= 15) and 22 obese controls (AHI<5) were studied. To confirm the diagnosis, all patients underwent standard polysomnography in our sleep disorders center. Serum samples were taken at 08:00 h in the morning after overnight fasting. Results: Significantly higher serum leptin levels were found in OSAS patients compared to controls (p=0.012), but there was no significant difference in serum ghrelin levels between OSAS patients and controls. Serum leptin levels were significantly correlated with body mass index in both OSAS patients (r=0.55, p=0.002) and controls (r=0.46, p=0.028), but only in OSAS patients was the leptin level significantly correlated with AHI (r=0.38, p=0.036). Conclusion: These data support findings suggesting that leptin is a hormonal factor affected by OSAS and not determined by obesity alone. Further studies are needed to investigate the relationship between serum ghrelin and OSAS. Copyright (C) 2005 S. Karger AG, Basel.