Sixty infants with acquired carbohydrate intolerance, 19 with protracted diarrhea, and 41 with features indistinguishable from acute infectious enteritis, were maintained on a lactose-free and cow milk protein-free formula for about eight weeks, after which jejunal biopsies were performed before and after oral provocation with cow milk protein. Following provocation, 43 infants showed mucocal abnormalities associated with marked reduction in the levels of all three disaccharidases. In one infant, mucosal change was unaccompanied by enzyme depletion. Six infants had enzyme depletion without visible mucosal changes. In ten infants the intestinal mucosa remained normal and there was no consistent pattern of change in the enzyme levels. The findings suggest that cow milk protein has a deleterious effect on the small bowel mucosa of young infants recovering from enteritis and may be an important contributing cause of acquired carbohydrate intolerance in these infants. In the management of young infants with acquired carbohydrate intolerance both the offending sugar and protein should be excluded.