We report a case of pseudocyst of the duodenal mesentery in an adolescent patient.
A 16-year-old man was admitted to the hospital because of 2-month history of left flank abdominal pain of increasing intensity. The patient had neither medical history nor abdominal injury. Abdominal computed tomography and magnetic resonance imaging revealed a unilocular cyst about 5.5 cm in diameter on the left side of the abdominal aorta. An enteric duplication cyst was diagnased, and a cystectomy by laparotomy was performed. The cyst had originated from the mesentery of the duodenum. The content of the cyst was milky white fluid. Microscopically, the cyst wall was made of fibrous tissue without epithelial lining, suggesting that it was a pseudocyst arising from the duodenal mesentery. Such pseudocysts of the mesentery are extremely rare, and there have been only 17 cases including ours in the Japanese literature.