This multicenter, prospective phase III-study is to compare the administration of ATG FRESENIUS to the NON-administration of ATG FRESENIUS in a myeloablative conditioning regimen followed by allogeneic hematopoeitic stem cell transplantation from an HLA-identical sibling in patients with acute Leukemia. This clinical trial is to show that the administration of ATG FRESENIUS reduces the risk of chronic Graft-versus-Host disease after allogeneic stem cell transplantation from HLA-identical siblings.