BACKGROUND This study investigates the relationship between serum concentration of RBP4 and visfatin and the metabolic syndrome. METHODS Patients with metabolic syndrome were studied between October, 2004 and September, 2005. All study subjects were aged 40 and over and lived in Taichung city, Taiwan. The Third Report of the National Cholesterol Education Program's Adult Treatment Panel (ATP III, 2001) was used to define the metabolic syndrome. Serum RBP4 and visfatin levels were measured by enzyme-linked immunosorbent assay. RESULTS Serum mean RBP4 levels in subjects who had all five abnormal components of metabolic syndrome (mean+/-SD=40.8+/-18.6) and those who had all components except hyperglycemia (43.5+/-23.5) were significantly higher than those in healthy controls (30.3+/-14.0 microg/ml) (p<0.05). Similar results were not found in serum visfatin levels. Using log-transformed serum RBP4 or visfatin levels as a dependent variable, we found that subjects with all five and four abnormal components of metabolic syndrome had significantly higher mean serum RBP4 levels (p=0.043 and 0.034, respectively), compared to healthy controls, after adjusting for other covariates. In contrast, similar results were not found in serum visfatin levels. CONCLUSIONS Metabolic syndrome is significantly associated with serum RBP4, but not serum visfatin.