To elucidate the electrocardiographic (ECG) characteristics of dilated cardiomyopathy (DCM), the authors analyzed the 12-lead ECGs and echocardiograms in 45 patients with DCM, 54 patients with left ventricular (LV) dilatation secondary to valvular heart disease (VHD), 101 hypertensive patients with LV hypertrophy, and 63 normal control subjects. In addition, serial ECG and echocardiographic changes in DCM during a mean follow-up period of 1.6 years were evaluated. Sokolow's criterion (S wave in lead V1 [SV1] + R wave in lead V5 or V6 [RV5 or RV6] > 35 mm) was met comparably in patients with DCM (69%), VHD (61%), and hypertension (74%) (P = NS). Notably, RV6 in DCM was the highest among the four groups and correlated with the degree of LV dilatation. In contrast, the R waves in leads I, II, and III (RI, RII, RIII) in DCM were the lowest and were not affected by the degree of LV dilatation, although RII and RIII in VHD and RI in hypertension correlated with the degree of LV dilatation and hypertrophy, respectively. As a result, all voltage ratios of RV6/RI, RII, RIII in DCM were not only the highest, but also increased linearly as the LV dilated progressively during the follow-up period. In particular, 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 correlated with ejection fraction. Subjects with DCM had a significantly higher RV6/Rmax than did patients with VHD, hypertension, and normal subjects (3.4 vs 1.7, 1.4, 1.2, respectively; P < .001), and this ratio of > or = 3 was seen in 67% of the DCM patients versus 4% of the VHD patients, 1% of the hypertensive patients, and 0% of the normal subjects. Thus, DCM commonly shows the ECG signs of LV hypertrophy, but characteristically has the high voltage ratios of RV6/RI, RII, RIII.