The aim of this study was to present anatomic macroscopic aspects and the relationship between the duodenum and the posterior abdominal wall. The authors describe anatomic types of peritoneal duodenal fossae and stress some points of surgical importance. Twenty-four cadavers, fixed in formalin, were dissected. Ten peritoneal fossae were given prominence and the authors show the anatomical structures topographically, from a superficial plane to a deep viscerae level. There is usually a complete fusion of the duodenal loop with the posterior parietal peritoneum except the duodenojejunal flexure. The study reveals three right retroduodenal fossae, three left retroduodenal fossae, two inferior duodenal fossae, one left paraduodenal fossa and one superior duodenal fossa. These peritoneal recesses mostly result from an incomplete adhesion of the Treitz's fascia. This work provides some explanation of paraduodenal hernias that represent a rare case of intestinal obstruction. Two cases of these internal hernias are illustrated and their pathophysiology and embryologic basis are discussed.