ExtractTen patients affected by congenital deficiency of intestinal sucrase and isomaltase activities were studied. The intraluminal a-amylolysis of amylopectin was found to be normal in these patients. It is incomplete during the 1st year of life, both in patients and in controls, as a consequence of the lower a-amylase activity in this age group.Maltotriose was well tolerated in two normal controls ages 32 and 3 months and in one 31-month-old patient: in these subjects an oral tolerance test with the trisaccharide was followed by a rapid increase of blood glucose (46-58 mg/100 ml) and did not cause diarrhea. However, in a 6-month-old patient fermentative diarrhea and low increase of blood glucose (24 mg/100 ml) followed oral load with maltotriose. In all patients, enzyme assays on intestinal biopsy specimens showed that the glucamylase activity of the heat-resistant maltase(s) was normal, while that of the sucrase-isomaltase complex was decreased.These results suggest that starch malabsorption in congenital sucrose and isomaltose intolerance results from deficiency of the enzymatic activities of the sucrase-isomaltase complex involved in starch digestion. SpeculationMalabsorption of starch can be expected to result from: (/) deficiency of pancreatic amylase; (2) congenital or acquired deficiency of the glucosidase activities of the brush border; (3) glucose malabsorption; (4) higher intestinal flow rate. The results in this study show that one particular form of starch malabsorption is a consequence of congenital deficiency of the activities of the sucrase-isomaltase complex involved in starch digestion. The methods used here should prove to be useful also in the investigation of other forms of starch malabsorption.