While both rats and mice develop pulmonary hypertension as a result of chronic hypoxia, mice show less pulmonary vascular remodeling compared to rats. Microarray gene analysis has demonstrated that lung osteopontin (OPN) gene expression was significantly elevated in hypoxic rats but was unchanged in hypoxic mice. Since OPN, which is a soluble secreted phosphoprotein, has been shown to play an important role in the proliferation of vascular smooth muscle cells, we hypothesized that OPN might be responsible for lung vascular remodeling. To test this hypothesis, we treated monocrotaline (MCT)-injected or chronically hypoxia-exposed rats with a peroxisome proliferator-activated receptor-γ ligand, pioglitazone, which has been reported to inhibit OPN gene expression in vitro, and assessed whole-lung OPN messenger RNA expression and pulmonary vascular wall thickening. Lung OPN gene expression was increased in nontreated MCT and hypoxia-exposed rats compared to that of controls. OPN was identified immunohistochemically in accumulated alveolar macrophages in the MCT rats and in thickened pulmonary arterial wall in the chronically hypoxic rats. Pioglitazone reduced lung OPN gene expression and pulmonary vascular thickening in both MCT and hypoxic rats. The administration of beraprost sodium, a stable prostacyclin analog, attenuated MCT-induced pulmonary vascular thickening, but did not affect the lung OPN gene expression. Then, we measured OPN gene expression in the lungs from three patients with primary pulmonary hypertension (PPH) undergoing lung transplantation. All of them were treated with prostacyclin IV. Two of these three subjects showed significantly higher lung OPN gene expression than did age-matched controls. Endothelial cells lining the slit-like blood spaces in plexiform lesions and accumulated macrophages in the PPH lungs were stained with an antibody to OPN. These findings indicate that OPN may be responsible for pulmonary vascular remodeling and could be a novel target for the treatment of PPH.