The matrix metalloproteinases (MMPs) and endothelin-1, a potent vasoconstrictor and mitogen for smooth muscle cells, have been shown to be involved in the pathogenesis of various vascular disorders. However, the expression of endothelin-1 and the activation of MMPs have not been fully evaluated in plexogenic pulmonary arteriopathy (PPA). Immunohistochemical and confocal microscopic studies were conducted to evaluate the reactivity of lung tissue from six patients with pulmonary hypertension for alpha-smooth muscle actin (alpha-SMA), desmin, vimentin, factor VIII, endothelin-1, various types of MMPs (MMP-1, MMP-2, MMP-3, MMP-7 and MMP-9), membrane type-MMPs (MT-MMPs), tissue inhibitors of MMPs (TIMPs), and type IV collagen. Four major arterial morphological abnormalities were recognized in PPA: muscularization of pulmonary arterioles, onion-skin lesions, cellular and mature plexiform lesions, and atheromas in elastic pulmonary arteries. Reactivity for MMP-2 and MT-1-MMP was found in endothelial cells and, to a lesser extent, in myofibroblasts proliferating in various lesions of PPA. Increased expression of endothelin-1 was observed in the latter cells and in endothelial cells. Some myofibroblasts were positive for MMP-3 and MMP-7 in the vascular lesions except for mature plexiform lesions. MMP-1, MMP-9 and TIMP-2 tended to be positive only in the atheromatous lesions. Staining for type IV collagen showed focal thinning and discontinuities of the endothelial basement membrane in plexiform lesions. This study demonstrates colocalization of MMP-2 with MT-1-MMP and increased expression of endothelin-1 in various arterial lesions of PPA. These changes may play important roles in the remodeling of arterial structures, particularly of basement membranes, in this disorder.