Acquired monosaccharide intolerance is characterized by the malabsorption of all carbohydrates, resulting in profuse acidic diarrhea. Five infants with this syndrome, ranging from two to six months of age, were studied by measuring their ability to absorb glucose given by intestinal perfusion. Jejunal biopsies obtained at the time of perfusion were studied by light and electron microscopy, and surface area measurements were obtained. Glucose absorption and surface area were significantly decreased (P less than 0.005) during the acute phase of the disease and improved during convalescence. A linear correlation (r = 0.8757) was observed between the ability to absorb glucose and intestinal surface area. It is concluded that the loss of absorptive surface is a major cause of the reduced carbohydrate absorption and consequent diarrhea.