Persistent diarrhoea with mucus-production developed in a 37 year homosexual man, and an initial diagnosis of ulcerative colitis was made after barium enema examination and rectal biopsy. The patient later developed cutaneous lesions which proved to be Kaposi's sarcoma, and the bowel lesion was also subsequently shown to be Kaposi's sarcoma. This tumour occurred as a manifestation of the acquired immune deficiency syndrome (AIDS). The patient was treated with alpha interferon, with partial regression of the skin lesions, but progression of the bowel tumour. Because of severe bowel symptoms, including episodes of subacute intestinal obstruction, the localised bowel disease was treated with radiotherapy. In view of the increasing incidence of AIDS, a diagnosis of Kaposi's sarcoma must be considered in homosexual men presenting with persistent diarrhoea, for which no infectious cause can be demonstrated.