From 1970 to 1983, eight patients who had severe right ventricular dilatation with little or no left ventricular abnormality were referred to our hospital. There were two men and six women, whose ages ranged from 16 to 38 years and averaged 27. Five patients were members of two families, which had definite hereditary histories of this disease. ECGs of seven patients suggested right ventricular abnormalities. Ventricular arrhythmias were documented in seven, whose ECGs suggested that the origins of the arrhythmias were in the right or both ventricles. Cardiothoracic ratios ranged from 0.50 to 0.66 and averaged 0.59. Five patients had wide respiratory splitting of the second heart sound. Echocardiography showed a marked increase in the right ventricular dimension. Right ventricular diastolic pressure was increased in three; left ventricular diastolic pressure was increased in two. Right ventricular angiography revealed marked dilatation and low ejection fractions of the right ventricle in all, but left ventricular function was nearly normal in them, except two patients who had minimal abnormality. The average follow-up period was 6.3 years and ranged from 3 months to 11 years. During the follow-up period, only two patients had ventricular tachycardia and three developed overt heart failure. Three patients died, two suddenly, and one from an attack of ventricular tachycardia. Autopsies of these two showed extreme dilatation of the right ventricle and one of them showed concentric hypertrophy of the left ventricle. Histological abnormalities were found in both ventricles. In summary, it is suggested that idiopathic dilated right ventricular cardiomyopathy can be classified in the spectrum of dilated cardiomyopathy.