The pulmonary vascular alterations of seven patients with idiopathic dilated cardiomyopathy (IDC) were morphometrically examined, and the relation between the vascular alterations and morphological status of the hearts was studied. Most patients with IDC showed not only fibrous thickening of small pulmonary veins but also intimal circumferential fibrosis and medial hypertrophy of small pulmonary muscular arteries to various degrees. The histological features of the pulmonary vessels were compatible with hypertensive vascular changes observed in patients with mitral stenosis. There was a significant correlation between medial hypertrophy of the pulmonary muscular arteries and right ventricular hypertrophy. The pulmonary vascular changes in IDC were always associated with left atrioventricular dilatation, but were only found in patients with prominent hypertrophy of the left ventricles. Medial hypertrophy of the pulmonary muscular arteries was related more to left ventricular hypertrophy than to left ventricular dilatation. These findings suggest that the pulmonary vascular changes in IDC are caused by venous pulmonary hypertension, which may be developed at the late stage when left ventricular hypertrophy predominates.