We analyzed lung specimens from 23 affected members of 13 families with known familial primary pulmonary hypertension. In this heritable disease, the pathogenetic basis of disease should be distinct within families, and the pathologic lesions should reflect the underlying biologic mechanisms. Two pathologic types of primary pulmonary hypertension have been proposed to exist, "plexogenic" and "thromboembolic." The purpose of this study was to determine the nature and variety of pathologic lesions in families with familial primary pulmonary hypertension and to discover whether the pattern of pathologic lesions found supported the existence of two types of disease. We found marked heterogeneity of pathologic lesions within and among families, including frequent coexistence of thrombotic and plexiform lesions. We conclude that the lesions found in primary pulmonary hypertension are not specific but represent different manifestations of the same pathologic process. Thus, plexiform and thrombotic lesions of the pulmonary arterioles may not arise from distinctly different biologic processes but may simply be different manifestations of the same process. Further insights in primary pulmonary hypertension must come from study of the biologic aspects of the intimal lesion.