Objective: Serum YKL-40 levels are elevated in patients with type 1 and 2 diabetes. However, the correlation between YKL-40 and gestational diabetes mellitus (GDM) remains unknown. The present study compared serum YKL-40 levels in pregnant women with GDM and those with normal glucose tolerance and evaluated the relationship between YKL-40 and insulin-resistant syndrome.;-;Methods: Thirty-five patients with GDM and 43 age-matched healthy pregnant women at 24-28 weeks of gestation were studied. In addition to anthropometric assessments, serum glucose, insulin, YKL-40, total cholesterol, triglycerides, high-density lipoprotein, low-density lipoprotein and glycated hemoglobin were measured in all subjects. All subjects underwent a 2-h 75-g oral glucose tolerance test (OGTT). Body mass index (BMI) and the homeostasis model assessment of insulin resistance (HOMA-IR) were calculated.;-;Results: Fasting and 2 h serum YKL-40 levels were significantly higher in pregnant women with GDM compared with controls (77.3 +/- 29.3 versus 50.9 +/- 16.7 ng/mL, p < 0.001, fasting concentrations; 63.5 +/- 20.1 versus 40.6 +/- 10.7 ng/mL, p = 0.009, 2 h concentrations). OGTT had no effect on YKL-40 levels in either group (p > 0.05). There were significant correlations between YKL-40 and glycated hemoglobin (beta = 0.37, p = 0.006), fasting insulin (beta = 0.49, p = 0.001) and HOMA-IR (beta = 0.18, p = 0.015) in the GDM group.;-;Conclusions: Serum YKL-40 levels are elevated in patients with GDM but are unaffected by OGTT. YKL-40 levels are related to glycated hemoglobin, fasting insulin and HOMA-IR. These results suggest that YKL-40 may be a major contributor to GDM.