The internal paraduodenal hernia is a most unusual disorder of the alimentary tract, due to a failure of the process of intestinal rotation. There exists a right and left variety, each of them presenting specific features. Often there are no symptoms or perhaps a vague abdominal malaise. Occasionnally one finds the clinical picture of obstruction, even of strangulation. The preoperative diagnosis is seldom made; however, it could be done if one keeps the possibility in mind. A successful surgical treatment necessitates an understanding of the mode of origin and a throughout knowledge of the anatomic variations.