Background The ¹¹C-acetate positron emission tomography can estimate myocardial oxidative metabolism, but previous studies have only evaluated small populations and the difference between ischemic (ICM) and idiopathic dilated cardiomyopathy (DCM) has not been fully investigated. The present aims were to evaluate global and regional myocardial oxidative metabolism in a well-characterized, large population with left ventricular (LV) dysfunction in order to clarify the metabolic differences between ICM and DCM. Methods and Results Seventy-eight patients with ejection fraction (EF) ≤50% (33 ICM; 45 DCM) were compared with 14 healthy controls. Myocardial oxidative metabolism was estimated with a clearance rate constant (K_mono) and the coefficient of variation (CV) of regional K_mono. Patients with LV dysfunction had reduced K_mono and higher CV (p<0.05). In the comparison of oxidative alterations with clinical variables there was a weak correlation between K_mono and LVEF (r=0.27). Although K_mono was reduced in both ICM and DCM, CV was more pronouncedly increased in ICM (p=0.001). In multivariate analysis, the presence of left bundle branch block (LBBB) was an independent predictor of heterogeneous oxidative metabolism in DCM (R²=0.30, p<0.0001). Conclusions Global reduction of myocardial oxidative metabolism occurred in both ICM and DCM. Heterogeneous oxidative metabolism was observed in these patients, especially those with ICM. Furthermore, LBBB was the independent predictor of heterogeneous oxidative metabolism in patients with DCM. (Circ J 2008; 72: 786 - 792)