PRIMARY and secondary intestinal disaccharidase deficiencies were first described in children 1; however, such enzyme deficiencies also occur in adults.2,3 We have previously reported4 the results of oral lactose tolerance tests in a group of healthy individuals, our ranges of normal intestinal disaccharidase and alkaline phosphatase activity, and the spatial location of these enzymes in the proximal small intestine. The present study concerns disaccharide tolerance tests and assays of intestinal disaccharidase and alkaline phosphatase activity in patients with various gastrointestinal diseases. Material and Methods Patients. —The 20 Caucasian patients in this study ranged in age from 27 to 79 years; two had an isolated lactase deficiency, four had undergone gastric surgery for benign peptic-ulcer disease, one had sucrose-isomaltose malabsorption, two had diarrhea with streatorrhea, one had regional enteritis, and ten had adult celiac disease. Methods. —The methods used for the tolerance tests, D-xylose excretion test, and stool fat