A 68-year-old man who had undergone radical prostatectomy for prostate cancer 5 years ago presented to our hospital complaining of an 8 kg weight loss within 2 months and a palpable periumbilical mass. A carbohydrate antigen 19-9 was noted to be increased to 85 U/mL, but other laboratory studies were unremarkable. An abdominal computed tomography scan showed a large spiculated mass with focal calcifications and surrounding inflammation along the small bowel mesentery (Fig. 1A and 1B). Positron emission tomography revealed an intense hypermetabolic mesenteric lesion (maximum standardized uptake value = 6.4) (Fig. 1C). An exploratory laparotomy was performed to obtain a differential diagnosis, especially to distinguish between a mesenteric malignancy and other benign conditions. Meticulous adhesiolysis and small bowel segmental resection with end-to-end anastomosis was performed. The gross specimen showed a yellowish hard mass forming fat infiltration along mesentery (Fig. 2A). Microscopically, the mass showed fat necrosis, sclerosing fibrosis, and focal infiltration of chronic inflammatory cells with fibroblasts without a granuloma, vasculitis, or malignancy (Fig. 2B). Based on histopathological results, he was diagnosed with sclerosing mesenteritis (SM), and following relief of symptoms he was discharged 10 days later, without any complications.