We prospectively analyzed the clinical, echocardiographic, and coronary arteriographic data of 51 patients with type 2 diabetes mellitus with left bundle branch block (LBBB), 51 patients with type 2 diabetes mellitus without LBBB, and 51 patients with isolated LBBB matched for age and gender. Extent of coronary artery disease (CAD) was classified according to the standard method into 1-, 2-, or 3-vessel disease and was estimated by calculation of the Gensini score. The left ventricular ejection fraction was analyzed by echocardiography. Age, gender, and percentage of patients with a smoking habit or family history of CAD did not differ among the groups. The rates of hypertension and levels of serum creatinine, cholesterol, and triglycerides were statistically higher in group I compared with the other 2 groups. Patients with diabetes and LBBB (group I) had significantly higher scores for the severity (Gensini score) of CAD (p <0.001) and more 3-vessel disease (p <0.001). After adjustment for hypertension, hypertriglyceridemia, and hypercholesterolemia with covariance analysis, the presence of LBBB was also associated with a higher Gensini score in patients with diabetes compared with those with diabetes but without LBBB and those with isolated LBBB (p <0.001). The present study, for the first time, has shown that patients with type 2 diabetes mellitus and concomitant LBBB have more severe and extensive CAD and advanced left ventricular dysfunction compared with those with diabetes but without LBBB and those with isolated LBBB.