It is often difficult to diagnose between cardiomegaly due to essential hypertension (EH) and that due to idiopathic cardiomyopathy (ICM). For the purpose of finding some useful clinical data for differentiating between cardiomegaly due to EH and that due to ICM, we examined the E.C.G., chest X-P and U.C.G. findings of 66 cases of EH and 46 cases of ICM. Body surface isopotential mappings (maps) were studied in 16 cases of EH and 18 cases of hypertrophic cardiomyopathy (HCM). Among those showing ASH or SH patterns in U.C.G., the ages of the EH group were greater than those of the ICM group. The thickness of the intraventricular septum (IVST) and the IVST/PWT ratio were greater and the left ventricular diastolic dimenaion (LVDd) was smaller in the ICM group than in the EH group. Among EH patients, the age and the systolic blood pressure were greater in those with abnormal U.C.G. findings. The septal activation time (SAT) obtained from maps correlated well with the IVST (r=0.55, p<0.005) and IVST/PWT ratio (r=0.61, p<0.005) obtained from U.C.G. In all of the HCM cases, the values os SAT were over 30msec., the values of IVST over 25mm and the IVST/PWT ratio over 2.0. These results suggest that these parameters should discriminate cardiomegaly due to EH from that due to ICM.