A 65-year-old otherwise asymptomatic man was found to have localized mesenteric thickening on an ultrasound examination during preoperative evaluation of a left inguinal hernia. Computed tomography also showed a localized mass-like lesion in the mesentery. Other than bulging, the patient had no abdominal symptoms, including no pain or signs of incarceration, related to the inguinal hernia. Therefore, follow-up observation was recommended. However, the abdominal mass continued to enlarge, so 1

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... ar 9 months later, a laparoscopic-assisted partial small bowel resection was performed. Histopathology showed inflammatory cell infiltration, particularly IgG4-positive plasma cells. Although IgG4-related disease was suspected, serum IgG4 levels were within normal limits. However, a probable diagnosis group of IgG4-related sclerosing mesenteritis was made based on diagnostic criteria. This case involving a mass-like lesion in which a definitive diagnosis was difficult is reported. Key words:IgG4-related disease,sclerosing mesenteritis,probable diagnosis group