Pulmonary artery sarcomas (PAS's) are extremely rare malignant tumors that arise from the endothelial lining of the pulmonary arteries. On CT scans PAS's appear as intraluminal filling defects in the pulmonary arteries, mimicking pulmonary embolism (PE). Due to the similarities in radiographic features as well as in clinical presentation, PAS's are usually misdiagnosed as pulmonary embolism. Since PASs are F-18 FDG avid, F-18 FDG PET/CT scan is a useful imaging tool for differentiating between these two conditions, as shown in this case report. We report a case of a 60-year-old woman presented with a 6-month history of chest pain, dyspnea on exertion, non-productive cough and weight loss. The initial CT pulmonary artery angiography showed extensive intraluminal mass in the pulmonary trunk and left pulmonary artery, diagnosed as massive pulmonary embolism. Since there was no clinical improvement after anticoagulant therapy, CT pulmonary angiography was repeated, and with no change observed in the intraluminal filling defect in pulmonary trunk, the possibility of tumor was raised. For further evaluation of a possible malignancy, F-18 FDG PET/CT was performed. It showed increased FDG uptake, suspicious for an aggressive tumor, in the intraluminal lesion of the pulmonary trunk and along the wall of the left pulmonary artery. There was no extrathoracic abnormality seen on PET/CT scan. Histopathological finding after complete pulmonary artery resection showed high grade undifferentiated pleomorphic sarcoma. F-18 FDG PET/CT is a useful tool for differentiating between pulmonary embolism and malignant intraluminal mass, and at the same time it enables the proper staging of the malignancy. < p > < /p >.