A tabulation of known unusual conditions in the duodenum is evidence that the theme is a large one. Included in the list are (1) conditions resulting from misdirected developmental processes, and (2) conditions resulting from injury or disease. In the first group are anatomic variations of all types, including abnormal position, mobility, blood supply and peritoneal relations of the organ, various forms of complete or partial obstruction, and primary diverticula. In the second group are obstructions caused by foreign bodies, impacted gallstones, or intestinal parasites, acquired diverticula, and both benign and malignant tumors of the duodenum. Such lesions are not only of academic interest, they are also of practical importance in the diagnosis and treatment of obscure, and sometimes apparently simple diseases. I have selected for brief consideration here three unusual conditions; membranous obstruction of the lumen of the duodenum in an adult, diverticula, and carcinoma of the duodenum. MEMBRANOUS