The correlation between vascular changes and arrhythmia in idiopathic cardiomegaly has been paid a little attention. We have reported a case of familial idiopathic cardiomyopathy, having various ECG abnormalities in many of her family tree. A 59 years old woman had a dyspnea and pointed out cardiomegaly in 1969. She lost consciousness, got hemiplegia and admitted in 1975. The ECG showed atrial standstill, idioventricular rhythm and multifocal VPC's. The congestive heart failure developed and she died of bronchopneumonia. Autopsy findings are as follows; The heart weighed 700g, showing dilatation of four chambers, focal endocardial fibrosis of right ventricle and scattered foci of intramural fibrosis in antero-septal and lateral wall. Extramural coronary arteries were unremarkable. The myocardial cells showed slight hypertrophy and disarray. Small arteries showed peculiar thickening of media and luminal narrowing. The conduction system showed marked atrophy and fibrosis, with a loss of conduction cells more than 70 percent in sinus node and about a half in His bundle. A-V node was well preserved. Posterior ramus of left bundle branch showed marked edema and/or fibrosis in bifurcation. We concluded that specific arterial changes of conduction system in this case might be related to the damage in conducting cells, resulting various ECG abnormalities.