Pulmonary hypertension is caused largely by an increase in pulmonary vascular resistance and is most frequently secondary to chronic pulmonary obstructive or interstitial diseases, recurrent pulmonary emboli, or antecedent heart diseases. Primary pulmonary hypertension (plexogenic pulmonary arteriopathy) is a rare disease and diagnosed only when the underlying causing factors are undetermined. Two autopsy cases of primary pulmonary hypertension, first documented at Taipei Veterans General Hospital, are reported. They showed apparent pulmonary vascular changes including medial hypertrophy, intimal proliferation and fibrosis and plexiform lesions of the muscular pulmonary arteries. The plexiform lesion has been considered to be characteristic in the histopathological diagnosis of primary pulmonary hypertension when there are no other associated diseases, such as cirrhosis of liver, pre- or post- tricuspid congenital cardiac shunts and portal vein thrombosis. The pathogenesis of the plexiform lesion is obscure. In the present report, we shall emphasize pathological changes of the "plexiform lesions" and discuss their pathogenesis.