Arterial leiomyosarcomas account for up to 21% of vascular leiomyosarcomas, with 56% of arterial leiomyosarcomas occurring in the pulmonary artery. While isolated cases of primary pulmonary artery leiomyosarcoma document survival up to 36 months after treatment, these uncommon, aggressive tumors are highly lethal, with 1-year survival estimated at 20% from the onset of symptoms. We discuss a rare case of a pulmonary artery leiomyosarcoma that was originally diagnosed as a pulmonary embolism (PE). A 72-year-old Caucasian female was initially diagnosed with 'saddle pulmonary embolism' based on computerized tomographic angiography of the chest 2 months prior to admission and placed on anticoagulation. Dyspnea escalated, and serial computed tomography scans showed cardiomegaly with pulmonary emboli involving the right and left main pulmonary arteries with extension into the right and left upper and lower lobe branches. An echocardiogram on admission showed severe pulmonary hypertension with a pulmonary artery pressure of 82.9 mm Hg, and a severely enlarged right ventricle. Respiratory distress and multiorgan failure developed and, unfortunately, the patient expired. Autopsy showed a lobulated, yellow mass throughout the main pulmonary arteries measuring 13 cm in diameter. The mass extended into the parenchyma of the right upper lobe. On microscopy, the mass was consistent with a high-grade primary pulmonary artery leiomyosarcoma. Median survival of patients with primary pulmonary artery leiomyosarcoma without surgery is one and a half months, and mortality is usually due to right-sided heart failure. Pulmonary artery leiomyosarcoma is a rare but highly lethal disease commonly mistaken for PE. Thus, we recommend clinicians to suspect this malignancy when anticoagulation fails to relieve initial symptoms. In conclusion, early detection and suspicion of pulmonary artery leiomyosarcoma should be considered in patients refractory to anticoagulation, prompting initiation of early intervention.