Pulmonary tumour thrombotic microangiopathy (PTTM) is characterised by wide spread tumour emboli along with fibrocellular intimal proliferation and thrombus formation in small pulmonary arteries and arterioles. PTTM is a rare but fatal complication of carcinoma, but the pathogenesis remains to be clarified. An autopsy case of PTTM caused by gastric adenocarcinoma is described, in which tumour cells in the PTTM lesion had positive immunoreactivity for platelet-derived growth factor (PDGF) and PDGF receptor (PDGFR), and proliferating fibromuscular intimal cells also showed expression of PDGFR. In addition, the overexpression of PGDF was detected in the alveolar macrophages. These findings suggest that PDGF derived from alveolar macrophages and from tumour cells may act together in promoting fibrocellular intimal proliferation. To the best of the authors' knowledge, the possible involvement of activated alveolar macrophages in PTTM has not been previously reported.