Retinol-binding protein 4 (RBP-4) has been reported to be associated with visceral-fat accumulation and parameters of the metabolic syndrome (MetS). In this study, we investigated the relationship between RBP-4, visceral fat, and the MetS during pronounced weight loss after bariatric surgery. Thirty-six subjects were examined before and 2 years after surgery. Abdominal-fat distribution was determined by ultrasound, metabolic parameters, and serum RBP-4 levels by standard methods. After surgery BMI decreased by 9.07 kg/m(2), visceral-fat diameter (VFD) decreased by 60.6%, and RBP-4 serum levels by 16.6%. Change of RBP-4 levels was associated with reductions of waist (r = 0.364, P = 0.037), waist-to-hip ratio (WHR) (r = 0.415, P = 0.016), and VFD (r = 0.425, P = 0.010). MetS, as defined by International Diabetes Federation (IDF), was present in 19 patients at baseline and in nine patients at follow-up. Change in RBP-4 levels was the best predictor for the diagnosis of MetS at follow-up. In the subgroup without MetS at baseline, the decrease in RBP-4 levels (-28.1% vs. -6.3%, P = 0.020) and reduction in VFD (-66.9% vs. -55.0%, P = 0.038) were significantly greater compared to the subgroup with MetS. We demonstrate a marked decrease of RBP-4 levels after bariatric surgery, which correlates with reduction in visceral-fat mass. Furthermore, the extent of changes in RBP-4 levels differs according to the severity of the MetS.