The 12-h urinary elimination of orally ingested mannitol and lactulose was studied in 29 infants aged 3 days-20 months admitted in an intensive care unit of gastrointestinal surgery, and in 24 age-matched healthy controls. In patients with short bowel syndrome, there was a significant fourfold reduction in urinary mannitol elimination compared with controls. The lactulose/mannitol urinary ratio was significantly increased in patients with necrotizing enterocolitis (p less than 0.01) and in patients with disorders of intestinal propulsion during episodes of diarrhea with a monomorphic fecal flora (p less than 0.01). The urinary recovery of mannitol and lactulose after oral loads is a good marker of intestinal mucosal damage in infants with surgical disorders of the digestive tract.