INTRODUCTION: Pulmonary arterial aneurysms (PAA) are a rare finding with most located proximally and are most commonly due to congenital heart disease and collagen vascular disease. We present a case of a patient presenting to interventional pulmonary clinic for biopsy of right sided mass seen on outside hospital CT found to be a large pulmonary arterial aneurysm. CASE PRESENTATION:A 68 y/o man presents to the interventional pulmonary clinic with a right lower lobe nodule incidentally noted on a CT A/P that measured 18mm. He has past medical history of hepatitis C, hypertension, hyperlipidemia, and prior abdominal surgeries for complicated diverticulitis. He has a smoking history of 45 pack years and quit about 9 years ago. His mother had lung cancer but otherwise has no personal history of cancer. He used to work as a welder and had exposure to asbestos. He takes medications for his hypertension and hyperlipidemia and takes aspirin for unclear reasons. His physical exam was unremarkable with normal vital signs and no clubbing, rashes, arthritis, or murmurs. An outside hospital radiology read of his CT A/P stated likely hamartoma vs carcinoid and less likely to be aneurysm. He underwent a PET scan showing the lesion with SUV max of 2.3 and a CT pulmonary artery protocol was performed which showed uniform contrast opacification of this lesion extending from a lobar PA confirming the diagnosis of a PA aneurysm. Interestingly this lesion was seen on a non-contrast CT of his chest done 20 years prior and given lack of change no interventions were deemed necessary at this time and close follow up was arranged.DISCUSSION: PAA is a rare entity and is usually a result of congenital heart disease. Acquired PAA can be due to rare collagen vascular diseases like Ehlers-Danlos syndrome and Marfan syndrome. Mycotic aneurysms can be caused by syphilis, tuberculosis, and many bacterium and fungi. Vasculitidies like behçet syndrome and Hugh-Stovin syndrome are also associated with PAA. Severe pulmonary artery hypertension can cause proximal PAA. Iatrogenic trauma to the pulmonary artery from PA catheters or surgeries is another cause. In our patient, given his age, stability, and lack of symptoms, we presumed his PAA was idiopathic or possibility a congenital defect and no further interventions were undertaken. Pseudoaneurysms or rapidly expanding aneurysms would need to be intervened upon due to risk of rupture, however given his stability this was deferred. CONCLUSIONS:It is important to be aware of pulmonary artery aneurysms as they can masquerade as a pulmonary mass and can sometimes be missed if IV contrast is not applied for a study. Secondary causes should be sought out and if the patient is symptomatic or having hemoptysis, coil embolization should be pursued.