0. 1). All the GI-GVHD and CMV colitis patients presented with a variety of colonic mucosal lesions. Besides the tortoiseshell-pattern mucosa and deep ulcer were characteristic lesions in GI-GVHD and CMV colitis respectively, the remaining mucosa lesions including edema, reddish patchy, erythma, erosion and superficial ulcer could not differentiate GI-GVHD from CMV colitis. Three GI-GVHD cases presented with pseud-omembrane, and 1 CMV colitis patient with herpes-like mucosa. Oozing bleeding of terminal-ileum mucosa and ileocecal valve inflammation could easily be found in GC patients. 63. 8% tissue samples were taken biopsies from rectosigmoid in GI-GVHD, and 70. 0% and 43. 8% in CMV colitis and GC patients respectively. Conclusion The positivity of peripheral blood CMV-DNA can not distinguish GI-GVHD from CMV colitis in allo-HSCT patients. GI-GVHD and CMV colitis manifest with a variety of lesions in colonoscopy, the tor- toiseshell-pattern mucosa in GI-GVHD and deep ulcer in CMV colitis are characteristic lesions. The patients of GI-GVHD complicated with CMV colitis readily present oozing bleeding of terminal-ileum mucosa and ileo-cecal valve inflammation. Colonoscopy and tissue biopsy of left-colon can diagnose the most of GI-GVHD and CMV colitis, but it's better to undertake pan-colon as well as terminal ileum examination for more accurate diagnosis.