CYSTS of the duodenum are rarely reported in the literature. Twelve cases of enterogenous cysts of the duodenum have been described since the initial case of S盲nger and Klopp,1 in 1880. A cyst of the ampulla of Vater, described by Brooks and Weinstein,2 appears to be one of the mucosa covering the ampulla of Vater, and yet it was both lined and covered with normal duodenal mucous membrane so that, as the authors described, this cyst was a congenital anomaly of the duodenal wall. Microscopic study of this cyst was not reported. These authors were unable to find a similar condition in the literature except the anomalous duodenal pouch presented by Grant,3 a pouch of mucosa which "hung within the lumen of the gut like a hollow polypus whose mouth was directed towards the pylorus." This pouch showed a reduplication of the entire mucous membrane of the