An extreme diminution of the absorption surface by resection or exclusion of the small intestine leads to the clinical picture of the short-intestine-syndrome. The absorption effect of the proximal small intestine can be exactly established with the help of the modified D-xylose-test. On the basis of three typical examples the high evidence of this functional test can be demonstrated. The pathophysiological principles of the intestinal adaptation after by-pass-operations and resections of the small intestine are discussed.