The cellular composition of pulmonary arteries of control and chronically hypoxic hypertensive neonatal calves and young rats was assessed for the expression of leukocyte/macrophage markers (ie, CD45, CD11b, CD14, CD68, and MHCII). Cells in the pulmonary artery adventitia of control animals did not express these markers, whereas in the pulmonary arteries of animals with hypoxia-induced pulmonary hypertension > 50% of adventitial cells expressed leukocyte/monocyte markers. Our assumption that these were blood-borne cells was supported by experiments on rats with hypoxia-induced pulmonary hypertension, in which in vivo 1-1′-dioctodecyl-3,3,3′,3′-tetramethyl indocarbocyanine perchlorate-labeled circulating monocytes/macrophages were identified in pulmonary artery adventitial lesions. A nonresident origin of these cells was further supported by organ and primary cell culture experiments in which hypoxia failed to induce the expression of leukocyte markers in resident pulmonary artery fibroblasts of control animals. Notably, double-label immunostaining and confocal microscopy analysis demonstrated that, in animals with hypoxia-induced pulmonary hypertension, most of the macrophage-like cells in pulmonary artery adventitia coexpressed a fibroblast marker, procollagen I, and thus exhibited a dual, macrophage-fibroblast phenotype. In addition, some cells coexpressed macrophage markers and α-smooth muscle actin, which raises the possibility that some myofibroblasts in the adventitia originate from the circulation. Furthermore, in vitro experiments showed that, compared to controls, cultured peripheral blood mononuclear cells from animals with hypoxia-induced pulmonary hypertension adhered in greater numbers, acquired fibroblast-like morphology, and secreted collagen I and extra-domian-A fibronectin, and some expressed α-smooth muscle actin.