Histopathologic and morphometric studies of small pulmonary arteries were performed in 16 cases of secundum atrial septal defect (ASD) and severe pulmonary hypertension. Besides typical plexogenic arteriopathies found in six cases, organized microthrombi and what we call "musculoelastosis," that is, proliferation of longitudinal smooth muscle bundles and elastic fibers, were seen in small pulmonary arteries in three and four cases, respectively. These changes were observed co-existing in the remaining three cases. Thrombi of the small pulmonary arteries and musculoelastosis were the forms of pulmonary vascular disease characteristically found in the older patients with both ASD and pulmonary hypertension. Yamaki's index of pulmonary vascular disease, though effective in describing severity of plexogenic arteriopathy, proved to be less so for the intimal lesions in old microthrombi and for musculoelastosis. Among the cases with plexogenic arteriopathy, there was a positive significant correlation between the medial thickness of small pulmonary arteries and peak arterial pressure, which, however, was not demonstrated when all the cases of ASD were included.