Sclerosing mesenteritis is a chronic and extremely rare inflammatory process that predominantly affects the small bowel mesentery. However, the peripancreatic region, the omentum and the retroperitoneum can also be involved. The etiology and pathogenesis of the disease are as yet unknown, but autoimmune disorders, previous abdominal surgery, trauma, ischemia and drugs could play a role. The clinical picture is nonspecific and varies from asymptomatic to diffuse abdominal complaints. Radiologic work-up and histologic evaluation of a biopsy specimen usually do not reveal typical findings and therefore establishment of the correct diagnosis is challenging. Owing to the rarity of the disease, there is no consensus on optimal pharmaceutical treatment and most patients are treated empirically. Surgical therapy is limited to biopsies for histologic confirmation of the tumor and management of complications. We report a case of a 51-year-old man with a large retroperitoneal mass. Magnetic resonance angiography showed a 9.7 x 7.7 x 5.9 cm tumor above the aortic bifurcation with encasement of the aorta and the vena cava. CT-guided biopsy was inconclusive but surgical biopsy provided a histologic diagnosis of sclerosing mesenteritis. The operative and postoperative course was uneventful and therapy with prednisolone and azathioprine was started. At 6 months' follow-up, the patient was in good condition and asymptomatic. Sclerosing mesenteritis, though a rare entity, should be included in the differential diagnosis when a patient presents with a mesenterial or retroperitoneal tumor.