BACKGROUND: Obstructive sleep apnea syndrome (OSAS) is associated with impaired glucose metabolism and insulin resistance. Retinol-binding protein-4 (RBP-4) is an adipokine, hypothesized to induce insulin resistance.OBJECTIVES: The aim of the study was to explore the association between serum RBP-4 levels and OSAS severity in nondiabetic, adherent to therapy OSAS patients and to investigate the role of continuous positive airway pressure (CPAP) in the alteration of RBP-4 levels.METHODS: OSAS patients (n = 62) without comorbidities or medication use were included. Fasting RBP-4, glucose and insulin levels, HbA(1c), homeostatic model assessment of insulin resistance index and lipid profile were measured at baseline and after 6 months of CPAP use. Patients were divided into group A (with fasting glucose levels <110 mg/dl, n = 47), and group B (with impaired fasting glucose (IFG), i.e. fasting glucose levels ≥110 mg/dl, n = 15).RESULTS: RBP-4 levels were not associated with apnea-related indices, anthropometric characteristics or markers of glycemic control, insulin resistance or lipid profile. In group A (but not in group B), a significant reduction was observed in RBP-4 (p = 0.046), HbA(1c) (p = 0.005), LDL cholesterol (p = 0.034), and high-sensitivity C-reactive protein (hs-CRP, p = 0.033) levels after 6 months of CPAP use.CONCLUSIONS: RBP-4 levels were not correlated with sleep, anthropometric characteristics, markers of glycemic control and insulin sensitivity. OSAS patients without IFG respond well to CPAP use as evidenced by the significant reduction in RBP-4, HbA(1c) and, additionally, hs-CRP and LDL- cholesterol levels. This treatment effect is not observed in patients with IFG.