We present a case of primary angiosarcoma of the pulmonary trunk that was initially misdiagnosed as a subacute massive pulmonary thromboembolism in a 26-year-old woman. This is an extremely rare disease that is usually indistinguishable from acute or chronic thromboembolic disease of the pulmonary arteries because the clinical and radiologic findings of pulmonary artery angiosarcoma are similar to those of pulmonary thromboembolism. Although the incidence of pulmonary artery angiosarcoma is very low, our case demonstrates that this disease entity should be included in the differential diagnosis of pulmonary thromboembolism, especially in patients who do not respond to anticoagulant therapy or present with no identifiable source of thromboembolic events.