OBJECTIVE: Fibroblast growth factor-21 (FGF-21) is a potent metabolic regulator, which in animal models has been shown to improve glucose metabolism and insulin sensitivity. Recently, FGF-21 was shown to be expressed and secreted from murine muscle cells in response to insulin stimulation. RESEARCH DESIGN AND METHODS: We studied muscular FGF-21 expression and plasma FGF-21 after acute insulin stimulation in young healthy men during a hyperinsulinemic-euglycemic clamp. Furthermore, we investigated systemic levels and muscle FGF-21 expression in humans with or without insulin resistance and chronic elevated insulin. RESULTS: FGF-21 was barely detectable in young healthy men before insulin infusion. After 3 or 4 h of insulin infusion during a hyperinsulinemic-euglycemic clamp, muscular FGF-21 expression increased significantly. Plasma FGF-21 followed the same pattern. In individuals with chronic elevated insulin, muscular FGF-21 expression was associated with hyperinsulinemia in men but not in women. In plasma, hyperinsulinemia and fasting glucose were positively associated with plasma FGF-21 while plasma FGF-21 correlated negatively with HDL cholesterol. No associations between muscle and plasma FGF-21 were found in the individuals with chronic hyperinsulinemia. CONCLUSIONS: FGF-21 is expressed in human skeletal muscle in response to insulin stimulation, suggesting that FGF-21 is an insulin-regulated myokine. In support, we found an association between chronic hyperinsulinemia and levels of FGF-21.