Although activated macrophages infiltrate the heart and adipose tissue in type 2 diabetes (T2D), the role of these cells in the genesis of cardiomyopathic phenotype is unknown. We tested the hypothesis that activated resident macrophages are fundamental mediators of diet-induced diabetic cardiomyopathy. Transgenic mice expressing an AP20187-inducible Fas-dependent suicide gene in macrophages and dendritic cells (MAFIA mice) were fed a high fat diet (HFD, 45% kcal fat) or control diet (CD, 10% kcal fat) for 28 weeks to induce T2D, and subsequently given either vehicle or AP20187 (0.5mg/kg IP every 10 d) for one month to induce macrophage apoptosis and depletion. HFD-fed mice exhibited a significant (p < 0.05) 43% increase in body weight when compared to CD fed mice (43±5 vs 35±1 g) and developed hyperglycemia (199±44 vs 121±10 mg/dL), hyperinsulinemia (1.27±0.6 vs 0.43±0.14 µg/L) and decreased insulin sensitivity indicating T2D. Echocardiography revealed concentric LV hypertrophy in HFD-fed mice with increased wall thickness (anterior wall 1.2±0.1 vs 0.9±0.1 mm), decreased LV end-diastolic diameter (LVEDD 3.9±0.2 vs 4.2±0.4 mm), and unchanged LV ejection fraction. AP20187 treatment significantly reduced the caloric intake (0.3±0.08 vs 0.4±0.05 kcal/g/day) and body weight of HFD-fed mice (36±2 vs 47±5 g), attenuated hyperglycemia (108±8 vs 195±14 mg/dL) and hyperinsulinemia (0.6±0.2 vs 1.9±0.5 µg/L), and improved insulin sensitivity when compared to vehicle treated HFD-fed mice. Importantly, the improvements in metabolic parameters were accompanied by reversal of LV remodeling with decreased wall thickness (anterior wall 0.8±0.1 vs 1.3±0.1 mm) and increased LVEDD (4.6±0.5 vs 3.9±0.2 mm). White adipose tissue weights in AP20187-treated HFD-fed mice were also significantly decreased when compared to vehicle treated HFD-fed mice [epididymal (0.6±0.2 vs 1.4±0.3 g); inguinal (0.6±0.2 vs 1.1±0.3 g); retroperitoneal (0.8±0.5 vs 1.4±0.4 g). We conclude that resident activated macrophages play central and causative roles in the development of diet-induced T2D and diabetic cardiomyopathy. Targeting systemic macrophage activation may represent a therapeutic approach to reverse pathological cardiac remodeling associated with T2D and obesity.