We hypothesized that clinical risk factors could be identified within 2 weeks of onset of severe (stage 3 or 4) acute gut GVHD for identifying a patient population with a very poor outcome. Among 1,462 patients who had allogeneic hematopoietic cell transplantation (HCT) between January 2000 and December 2005, 116 (7.9%) developed stage 3–4 gut GVHD. The median time to onset of stage 3–4 gut GVHD was 35 (4–135) days after allogeneic HCT. Eighty-five of the 116 patients (73%) had corticosteroid-resistance before or within 2 weeks after the onset of stage 34 gut GVHD. Significant risk factors for mortality included corticosteroid-resistance (HR=2.93; p=0.0005), age >18 years (HR=4.95; p=0.0004), increased serum bilirubin (HR 2.53; p=0.0001), and overt gastrointestinal bleeding (HR 2.88; p=0.0004). Among patients with stage 3–4 gut GVHD, the subgroup with 0, 1 or 2 risk factors had a favourable prognosis, whereas the subgroup with 3 or 4 risk factors had a dismal prognosis. This information should be considered in designing future studies of severe gut GVHD and in counseling patients about prognosis.