[14C]Sucrose absorption was studied in 4 healthy controls and 4 patients after jejunoileal bypass using an ileal perfusion technique which made it possible to distinguish 14C-absorption in the small bowel from that occurring in the colon. Healthy controls failed to absorb 2-4% of a 50-g dose of [14C]sucrose in the small bowel; however, virtually none of the [14C] appeared in feces in a dialyzable form with appreciable osmotic activity. In bypass patients, the small bowel failed to absorb 29-84% of the 50-g dose of [14C]sucrose. Approximately two-thirds of the nonabsorbed [14C] was in the form of sucrose and the remainder was nearly all present as monosaccharides. A mean of only 42% of the [14C] not absorbed in the small bowel appeared in the feces and only about one-third of this fecal [14C] was in a dialyzable form with appreciable osmotic activity. Thus, the colon plays an important role in carbohydrate malabsorption by salvaging carbohydrate and reducing osmotic activity of the nonabsorbed sugar. This colonic function appears to depend upon bacterial metabolism of the carbohydrate, and individual variations in diarrhea and weight loss associated with carbohydrate malabsorption could reflect individual differences in the bacterial flora of the colon.