Graft versus host disease affecting the large bowel causes destruction of the crypt epithelium. There is a selective sparing of enterochromaffin cells in the majority of cases. As a consequence, single as well as small clumps of enterochromaffin cells are to be seen in the sites formerly occupied by the destroyed crypt epithelium. The reason for this phenomenon is unclear, but it may be related to the fact that the enterochromaffin cells are end-stage and non-proliferating cells. This is useful diagnostically. However, cytotoxic drugs or irradiation must be excluded as the cause of the mucosal damage to bowel as there are theoretical reasons to expect that a similar phenomenon will be seen after these forms of therapy.