Fetuin-A (alpha2-Heremans-Schmid glycoprotein), a circulating glycoprotein, can inhibit insulin signaling both in vivo and in vitro. Recently, we and another independent group have shown that fetuin-A is positively associated with insulin resistance in humans. Furthermore, it has been reported that higher fetuin-A levels are associated with metabolic syndrome and atherogenic lipid profiles. These data suggest that fetuin-A might be a regulator of insulin resistance and/or metabolic syndrome. However, it is not clear how fetuin-A levels are regulated. To address this, we investigated the effects of representative insulin-sensitizing therapies such as pioglitazone, metformin, and aerobic exercise on fetuin-A levels. Twenty-seven patients with type 2 diabetes mellitus were divided into pioglitazone-treated (Pio), metformin-treated (Met), and exercise-treated (Ex) groups. Ten patients in the Pio group and 9 patients in the Met group took 15 or 30 mg/d pioglitazone or 500 or 750 mg/d metformin, respectively, for 6 months. Eight patients in the Ex group underwent a 3-month aerobic exercise program. Serum fetuin-A levels were measured before and after each intervention. Intervention significantly decreased hemoglobin A(1c) in all groups. After treatment, serum fetuin-A levels significantly decreased in the Pio group (291.2 +/- 57.7 to 253.1 +/- 43.9 microg/mL, P = .006), whereas there were no changes in serum fetuin-A after intervention in either the Met or the Ex groups. We hypothesize that pioglitazone could partially ameliorate insulin resistance via modulating fetuin-A levels.