We report a middle-aged woman who died 2 days after presenting with dyspnea and severe pulmonary hypertension of unknown etiology. Her symptoms were highly suggestive of pulmonary embolism, but clinical evaluations for that disease yielded negative results. Autopsy revealed a Krukenberg tumor of the left ovary, representing metastatic gastric carcinoma from an occult primary lesion. Although the lungs exhibited no gross evidence of pulmonary emboli or neoplasia, microscopic examination revealed diffuse microscopic metastases in the pulmonary arterial vasculature. The pulmonary arteries exhibited fibrocellular intimal proliferation with smooth muscle colonization of the luminal neoplastic lesions and associated microthrombi. This disease entity, known as tumor-related thrombotic pulmonary microangiopathy, results in generalized microvascular obliteration and subsequent pulmonary hypertension. It is a rare condition that is distinct from ordinary pulmonary thromboembolism and primary pulmonary hypertension. Tumor-related thrombotic pulmonary microangiopathy should be considered diagnostically by the autopsy pathologist in cases of rapidly evolving pulmonary hypertension in a middle-aged or elderly individual, or respiratory failure of unknown cause, especially if there is a history of a visceral malignancy.