[en] A previously healthy, non-smoking, 72-year-old woman was referred to our hospital with abnormal chest X-ray and chest discomfort. A 3.5-cm, well-defined, right paratracheal mass was revealed on non-contrast chest computed tomography (CT), which was suspected to be an azygous vein aneurysm, lymph adenopathy or neurogenic tumor. Whole-body integrated positron emission tomography/magnetic resonance imaging (PET/MRI) (Biograph mMR; Siemens Healthcare, Erlangen, Germany) was performed for the differential diagnosis. A homogenously enhancing mass connected with the azygous vein was well visualized in a post-contrast volumetric interpolated gradient echo (VIBE) sequence. Additionally, the PET showed minimal ¹⁸F-fluorodeoxyglucose (FDG) uptake (maximum standardized uptake value [SUVmax], 1.83), similar to that of the great vessels, with no filling defect to suggest thromboembolism and no significant FDG uptake to suggest active thrombo-embolism or malignancy. The imaging findings in integrated PET/MRI were useful to characterize azygous vein aneurysm. The patient had a video-assisted thoracoscopic surgery to relieve the symptom of chest pain. A congenital etiology of azygous vein aneurysm was confirmed by pathology