Diagnostic value of disaccharide tolerance tests in children. Acta Paediatr Scand, 64:693, 1975.--The diagnostic value of oral lactose and sucrose tolerance tests was investigated in 61 children. A total of 105 oral disaccharide tests were carried out. When the rise in blood sugar was low, the same disaccharide was, as a control measure, instilled directly into the small intestine through a tube. This was carried out in 40 cases. In 21 patients the rise in blood sugar following the two forms of administration was correlated with the disaccharidase activity in a peroral small-intestine biopsy. The incidence of false-positive oral lactose tests was between 23 and 30%, that of false-positive oral sucrose tests between 24 and 33%. A border value of 20 mg per 100 ml in the rise of blood glucose within the first hour following a direct intra-intestinal administration affords a very satisfactory distinction between patients with and without disaccharide malabsorption. Blood glucose determinations exceeding one hour were found to be without diagnostic value.