The plexiform lesion in primary pulmonary hypertension is a glomeruloid structure forming channels in branches of the pulmonary artery. These lesions have been considered an abnormal growth of modified smooth muscle cells. We present immunohistochemical evidence in 10 cases of plexogenic pulmonary hypertension that the plexiform channels and the concentric obliterative arteriopathy associated with these channels represent abnormal growth of factor VIII-related antigen-positive endothelial cells. In addition, these cells strongly expressed vimentin, a growth- and differentiation-related intermediate filament. Morphologically and immunohistochemically, the lesions resembled the neovascularization associated with the brain tumor glioblastoma multiform. Furthermore, we noted an exclusively perivascular inflammatory cell infiltrate (but no vasculitis) in seven of the 10 cases with plexogenic arteriopathy composed of T cells, B cells, and macrophages. Our findings indicate that the plexiform lesion may result from a deregulated growth of endothelial cells. The presence of perivascular inflammatory cells suggested that cytokines and growth factors may further influence the development of the plexiform lesion.