The authors hypothesized that serum retinol-binding protein 4 (RBP4) concentrations will be higher in gestational diabetes mellitus (GDM) subjects. This study tested both women with GDM and healthy pregnant women and correlated their serum RBP4 concentrations with body mass index (BMI) and a variety of other parameters. Also, since there is no information on the relationship between RBP4 concentrations in maternal and fetal serum, this study measured these at delivery and examined whether there were correlations between the cord serum RBP4 levels and maternal serum RBP4 concentrations, neonatal birth weights, and gestational age at delivery. A total of 40 women were evaluated: 20 women with GDM and 20 healthy pregnant women to serve as control subjects. Serum RBP4 concentrations were analyzed with the use of an enzyme-linked immunosorbent assay kit. Serum RBP4 concentrations at glucose challenge test (GCT) were significantly higher in the GDM group (42.4 +/- 13.8 ng/mL) than in the healthy control group (32.0 +/- 8.7 ng/mL; P = .007). BMI at GCT (P = .003) and GDM/no GDM (P = .014) were significantly correlated to serum RBP4 concentrations at GCT by multiple linear regression analysis. In GDM subjects, serum RBP4 concentrations immediately after delivery were significantly lower than those at GCT (30.1 +/- 11.0 ng/mL, 42.4 +/- 13.8 ng/mL; P < .001), but there was no such difference in normal subjects (30.9 +/- 10.0 ng/mL, 32.0 +/- 8.7 ng/mL; P = .581). Cord serum RBP4 concentrations were significantly lower than maternal serum RBP4 concentrations at delivery (10.9 +/- 3.8 ng/mL, 30.5 +/- 10.4 ng/mL; P < .001). Only fetal birth weight (P = .049) was independently related to cord serum RBP4 concentrations at delivery by multiple linear regression analysis. This study found increased serum RBP4 concentrations at GCT in GDM subjects, and GDM was significantly correlated to serum RBP4 levels after adjustment for the effect of BMI. Lower RBP4 concentrations were found at delivery in GDM subjects. Maternal serum RBP4 concentrations were significantly higher than cord serum RBP4 concentrations, and fetal birth weights were independently correlated to cord serum RBP4 concentrations. These findings may indicate that RBP4 plays a role in the pathogenesis of GDM. However, further experiments are required to clarify this role and find a possible regimen for GDM treatment.