Expected results: If some biomarkers are found significantly associated with diagnosis or prognosis of acute GVHD, they will be compared with the current clinical, biological and histological markers. Indeed, these markers have a clinical potential impact only if they give similar or better information than routine currently available markers, ie: clinical GVHD grading system, performance status, gut endoscopy and histology. The non-invasivity of these biomarkers should also be taken into account (in comparison to histology).