Five infants with intolerance to lactose or sucrose and one infant with intolerance to both lactose and sucrose have been studied. These infants were symptomatic with fermentative diarrhea, vomiting, and failure to gain weight. The tolerance of these patients to disaccharides was measured, and the sugar or sugars to which they were intolerant were removed from their diets. The infants began to gain weight as their diarrhea abated. After they were asymptomatic and thriving for a period of time, they were re-examined and were found to respond normally to the ingestion of disaccharides. Disacchariduria was detected in 5 of the 6 infants during the acute period of their disorders, but not at the time of their reexamination. This finding is probably correlated to the amount of disaccharide ingested and the activity of the specific intestinal disaccharidase at the time of the test. The clinical symptoms and signs of these patients with acquired intolerance to disaccharides were identical to those of patients with congenital absence of intestinal disaccharidases.