Mesenteric inflammation caused by complicated duplication cysts rarely occurs. We present and discuss the imaging findings of a mesenteric inflammatory mass caused by complicated duplication cysts in an infant. Sonography showed a complex, echogenic cystic mass occupying the right subhepatic region containing two thick-walled cystic lesions inseparable from the normal bowel loop. On CT images a large, heterogeneously enhancing, hypodense peritoneal mass was observed to infiltrate the mesentery, which contained small cystic masses with enhancing thick walls. At surgery a mesenteric inflammatory mass caused by two complicated duplication cysts was found to be firmly fixed by surface contact with the ileum. A review of the literature relating to the etiologic relationship between a mesenteric inflammatory mass and complicated duplication cysts is presented.