The term malabsorption is used to define the defective passage of the products of normal digestion through the intestinal mucosa. The clinical consequences and expression of this phenomenon are extremely variable. This paper discusses in detail those conditions which, due to frequency, clinical polymorphism and difficulty in diagnosis or in therapeutic control represent particularly current problems in the field of Internal Medicine. At the clinical level, physicians more and more frequently find a dissociation between malabsorption, intended as a chemical-physical phenomenon, enteropathy, intended as a morbid process characterized by evident morphological alterations of the small intestine, and malabsorption syndrome, intended as a combination of symptoms consequent to malabsorption. Diseases of the small intestine are thus more frequently accompanied by minor, transitory and extra-intestinal symptoms. This explains why many such diseases still remain underdiagnosed. Since the chief clinical problem involves diagnosis, intestinal absorption should always be appropriately assessed even if the symptoms do not at first sight appear to be connected with malabsorption.