Pulmonary tumor thrombotic microangiopathy (PTTM) is characterized by fibrocellular intimal proliferation and thrombus formation in small pulmonary arteries and arterioles in patients with metastatic carcinoma. Osteopontin (OPN) is a multifunctional cytokine and adhesive protein, and has been demonstrated to be implicated in fibrosis, neointima formation, arterial occlusion by thrombus, and tumor metastases in cooperation with platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF). Herein is described an autopsy case of gastric adenocarcinoma with severe pulmonary hypertension due to PTTM. Histologically, tumor cell emboli markedly induced both fibromuscular intimal thickening and thrombosis, resulting in luminal stenosis and occlusion of small pulmonary arteries and arterioles. Tumor cells, both in the PTTM lesions and primary gastric carcinoma, had positive immunoreactivity for OPN, PDGF, and VEGF. In addition, proliferating fibromuscular intimal cells also showed expression of OPN, PDGF, and VEGF. These findings suggest that OPN may be involved in fibrocellular intimal proliferation and thrombus formation in PTTM together with PDGF and VEGF. To the best of the authors' knowledge this is the first report to demonstrate the possible involvement of OPN in PTTM. It is postulated that OPN is one of the candidate molecules for the development of PTTM.