OBJECTIVE - Fibroblast growth factor (FGF)-21 is highly expressed in the liver and regulates hepatic glucose production and lipid metabolism in rodents. However, its role in the pathogenesis of type 2 diabetes in humans remains to be defined. The aim of this study was to quantitate circulating plasma FGF-21 levels and examine their relationship with insulin sensitivity in subjects with varying degrees of obesity and glucose tolerance. RESEARCH DESIGN AND METHODS - Forty-one subjects (8 lean with normal glucose tolerance [NGT], 9 obese with NGT, 12 with impaired fasting glucose [IFG]/impaired glucose tolerance [IGT], and 12 type 2 diabetic subjects) received an oral glucose tolerance test (OGTT) and a hyperinsulinemic-euglycemic clamp (80 mU/m2 per min) combined with 3-[3H] glucose infusion. RESULTS - Subjects with type 2 diabetes, subjects with IGT, and obese subjects with NGT were insulin resistant compared with lean subjects with NGT. Plasma FGF-21 levels progressively increased from 3.9 ± 0.3 ng/ml in lean subjects with NGT to 4.9 ± 0.2 in obese subjects with NGT to 5.2 ± 0.2 in subjects with IGT and to 5.3 ± 0.2 in type 2 diabetic subjects. FGF-21 levels correlated inversely with whole-body (primarily reflects muscle) insulin sensitivity (r = -0.421, P = 0.007) and directly with the hepatic insulin resistance index (r = 0.344, P = 0.034). FGF-21 levels also correlated with measures of glycemia (fasting plasma glucose [r = 0.312, P = 0.05], 2-h plasma glucose [r = 0.414, P = 0.01], and A1C [r = 0.325, P = 0.04]). CONCLUSIONS - Plasma FGF-21 levels are increased in insulin-resistant states and correlate with hepatic and whole-body (muscle) insulin resistance. FGF-21 may play a role in pathogenesis of hepatic and whole-body insulin resistance in type 2 diabetes.