A 57-year-old man presented to the emergency department with a chief complaint of progressive left lower quadrant abdominal and lower back pain for the past 3 weeks. Review of systems revealed only a history of constipation. There was moderate tenderness to palpation of the left abdomen on examination. Vital signs were normal. A complete blood count, basic metabolic panel, and liver function tests with lipase were normal except for platelets of 479,000/mm 3 and creatinine of 1.2 mg/dL (glomerular filtration rate 74 mL/min). Computed tomography (CT) of the abdomen and pelvis with intravenous contrast ( Figure 1) was performed, followed by bedside ultrasonography (US) of the aorta (Figure 2). The bedside US was performed to gain images of possible abnormal findings seen on aortic US. After identification of a mass surrounding the aorta and discussion with Vascular Surgery, the erythrocyte sedimentation rate and C-reactive protein were obtained, with values of 54 mm/hr and 6.2 mg/dL, respectively. The final diagnosis was based on imaging studies including magnetic resonance imaging (MRI). A biopsy of the tissue was not attempted due to the risk associated with biopsy. The patient was placed on high-dose prednisone (1 mg/kg/day), which resulted in the retroperitoneal fibrosis (RPF) shrinking by one-half after a month of treatment. RPF is a rare condition resulting in chronic inflammation and fibrosis of the tissue surrounding the aorta and adjacent retroperitoneal structures.