Observations over a period of 6 years are reported on 10 children in whom the diagnosis of congenital sucrase isomaltase deficiency (SID) was confirmed by quantitative estimation of disaccharidase activity of the small intestinal mucosa. Repeat biopsies were performed on eight of the ten patients and showed no evidence that sucrase isomaltase activity is acquired. Sucrose tolerance tests (2 gm/kg) showed no significant rise in blood glucose in the seven patients in whom they were performed. This condition may appear in young infants with severe diarrhea resulting in a malabsorption syndrome and failure to thrive. It may also be manifest in a milder clinical presentation with bothersome diarrhea in spite of normal growth and development in the older infant or young child. The diagnosis in this latter group can be difficult, and is frequently missed.