The aerobic and anaerobic bacterial flora of the duodenum were studied in 10 infants with transient monosaccharide intolerance. 5 infants had protracted diarrhoea after an episode of acute gastroenteritis and 5 had acute gastroenteritis. The duration of monosaccharide intolerance ranged from 1 to 20 days. Serial intubations were performed on 8 infants. In 5 of 8 cases the bacterial flora were of a normal type at the first intubation at 0--4 days after the onset of monosaccharide intolerance. In 4 of those 5 patients there was an increase in bacterial count a few days later and anaerobes appeared. These findings are not consistent with the suggestion that monosaccharide intolerance is caused by abnormal bacteria in the upper small bowel.