This randomized allogeneic transplantation protocol compares i.v. Treosulfan-based conditioning therapy with reduced intensity i.v. Busulfan-based conditioning in adult AML and MDS patients at increased risk for standard conditioning therapies. The protocol is based on results of previous phase I/II trials evaluating Treosulfan/Fludarabine conditioning prior to allogeneic haematopoietic stem cell transplantation. The reference arm (reduced intensity i.v. Busulfan/Fludarabine) is considered to be accepted medical practice for the study patient population.