To elucidate electrocardiographic differences of dilated cardiomyopathy (DCM) and coronary artery disease (CAD) with left ventricular (LV) dysfunction, 12-lead electrocardiograms in 23 patients with DCM, 36 patients with CAD and LV dysfunction, and 63 normal subjects were analyzed. Abnormal Q waves were seen in 69% of CAD patients and in 26% of DCM patients. Abnormal Q waves in leads II, III, aVF, or V2-V4 were present in 61% of CAD patients compared with 4% of DCM patients (P < .001). The R wave in lead RV6 voltage in DCM was the highest among the three groups and correlated with the degree of LV dilatation. RV6 voltage in CAD was the lowest and decreased in proportion to the severity of the apical thallium-201 defect RV6 voltage of 15 mm or more was present in 78% of DCM patients compared with 11% of CAD patients (P < .001). The R waves in leads I, II, and III (RI, RII, RIII) were also low in DCM. Therefore, all voltage ratios of RV6/RI, RII, RIII were the highest in DCM. In particular, the ratio of RV6 over the maximum R wave in leads I, II, and III (RV6/Rmax) in DCM correlated with the degree of LV dilatation and inversely with ejection fraction. RV6/Rmax was significantly higher in the DCM group compared with the CAD and control groups (3.3 vs 1.2, 1.2, respectively; P < .001). This ratio of 3 or more was present in 61% of the DCM patients but in none of the CAD patients or normal subjects (P < .001).(ABSTRACT TRUNCATED AT 250 WORDS)