Based on the data obtained during clinical examination of 1,026 patients with small intestinal diseases the authors provide the portion of laboratory and instrumental methods employed in the diagnosis of different disease entities. The clinical picture of small intestinal diseases is mainly determined by the gravity of malabsorption. Histological examination of the small intestinal mucosa is a method of choice in the diagnosis of gluten enteropathy, Whipple's disease, primary lymphangiectasis and amyloidosis. Immunoassays play the key role in recognition of variable immunodeficiencies and disease of heavy alpha-chains. Meanwhile in differential diagnosis of Crohn's disease, small intestinal tumors, congenital abnormalities of rotation and in some others, the leading part is played by x-ray methods. The authors describe the treatment schedule based on the pathogenetic approach, that makes it possible to reach a stable clinical remission and recovery of the working capacity even in part of patients with stage III malabsorption.