Acute graft versus host disease (GVHD) is the leading cause of morbidity and mortality after bone marrow transplantation. Target organs of acute GVHD are the skin, intestinal tract, and liver. Intestinal involvement in acute GVHD varies from anorexia, vomiting, abdominal pain, and diarrhea to massive gastrointestinal bleeding. These symptoms are not specific for acute GVHD. Thus, the diagnosis can be very difficult to establish on clinical grounds. We experienced a patient with bloody diarrhea successfully diagnosed as having acute intestinal GVHD on sigmoid colon biopsy. Endoscopic appearance of the rectum and sigmoid colon of patient with acute GVHD is presented. A 20-year-old man with chronic myelogenous leukemia suffered from bloody diarrhea 3 months after unrelated bone marrow transplantation. Sigmoidoscopy showed multiple erosion with spontaneous bleeding. The section of the biopsy specimen were compatible with acute GVHD. A follow-up endoscopy after steroid therapy showed gradual healing of the lesions.