CARBOHYDRATE intolerance due to congenital or acquired deficiency of intestinal disaccharidases has been recognized repeatedly since the studies of Dahlquist.1-3 Lactose,4,5 saccharose 6-9 and maltose intolerance,6 as well as a combination of saccharose and isomaltose intolerance,10-13 have been described in children as inborn errors of metabolism. It has been observed that children with a complete deficiency of a disaccharidase may, when they are older, show spontaneous clinical improvement.9 To explain this phenomenon the hypothesis of an adaptation to the incriminated disaccharide, due perhaps to an increase in activity of enzymes with normally a weak action, was put forward.9-13 This hypothesis might explain the apparent rarity of congenital disaccharidase deficiency in adults. Brief reports, without enzyme studies, have been published on congenital saccharase deficiency in an 18-yearold boy 14 and in the mother of three children with the same defect.15 Sonntag et al16described the only case of saccharose-isomaltose