Interesting findings were obtained in the lung biopsy of a 46 year old female patient with secundum atrial septal defect and pulmonary hypertension. In the intima of small pulmonary arteries, there were a marked increase in elastic fibers and marked proliferation of longitudinal smooth muscle cells. The vascular lumen was markedly stenotic, but reconstruction of small pulmonary arteries indicated that all vascular lumina were patent. The case was accordingly diagnosed as operable and closure of atrial septal defect was undertaken. Three years postoperatively, the patient is well with much improved condition and lowered pulmonary arterial pressure was demonstrated by cardiac catheterization. This case had apparently serious vascular lesions due to severe intimal thickening which we labeled "musculoelastosis", but it was found that such thickening alone did not cause peripheral pulmonary arterial occlusion. It is therefore thought to be a benign pulmonary vascular condition.