OBJECTIVE - Insulin-resistant states, such as obesity and type 2 diabetes, contribute substantially to accelerated atherogenesis. Null mutations of myostatin (Mstn) are associated with increased muscle mass and decreased fat mass. In this study, we determined whether Mstn disruption could prevent the development of insulin resistance, proatherogenic dyslipidemia, and atherogenesis. RESEARCH DESIGN AND METHODS - C57BL/6 Ldlr-/- mice were cross-bred with C57BL/6 Mstn-/- mice for >10 generations to generate Mstn-/-/Ldlr-/- double-knockout mice. The effects of high-fat/high-cholesterol diet on body composition, plasma lipids, systemic and tissue-specific insulin sensitivity, hepatic steatosis, as well as aortic atheromatous lesion were characterized in Mstn-/-/Ldlr-/- mice in comparison with control Mstn+/+/Ldlr-/- mice. RESULTS - Compared with Mstn+/+/Ldlr-/- controls, Mstn-/-/Ldlr-/- mice were resistant to diet-induced obesity, and had greatly improved insulin sensitivity, as indicated by 42% higher glucose infusion rate and 90% greater muscle [3H]-2- deoxyglucose uptake during hyperinsulinemic-euglycemic clamp. Mstn -/-/Ldlr-/- mice were protected against diet-induced hepatic steatosis and had 56% higher rate of hepatic fatty acid β-oxidation than controls. Mstn-/-/Ldlr-/- mice also had 36% lower VLDL secretion rate and were protected against dietinduced dyslipidemia, as indicated by 30-60% lower VLDL and LDL cholesterol, free fatty acids, and triglycerides. Magnetic resonance angiography and en face analyses demonstrated 41% reduction in aortic atheromatous lesions in Ldlr-/- mice with Mstn deletion. CONCLUSIONS - Inactivation of Mstn protects against the development of insulin resistance, proatherogenic dyslipidemia, and aortic atherogenesis in Ldlr-/- mice. Myostatin may be a useful target for drug development for prevention and treatment of obesity and its associated type 2 diabetes and atherosclerosis.