We report a 10-year-old male with Down's syndrome, who received a bone marrow transplant for acute lymphoblastic leukaemia. Subsequent acute graft-versus-host disease (GvHD) of the gut progressed to small bowel obstruction. At laparotomy, the small bowel appeared solid and contracted with no or minimal luminal patency. Although the caecum had a lumen, it was indistensible, and it was not possible to enter the terminal ileum. Histology of the obstructed bowel showed extensive necrosis of the mucosa, muscularis mucosa and submucosa of most of the small bowel wall, causing obliteration of the lumen. The changes were presumed to be related to post inflammatory atrophy. This extreme manifestation of GvHD could thus be called obliterative enteritis. Both cytomegalovirus and adenovirus were isolated from the patient. These viruses may have contributed to the severity of the intestinal GvHD.