This study examines whether the addition of decitabine to the standard Flu/TBI conditioning regimen prior to allogeneic stem cell transplantation in poor and very poor risk AML patients, reduces the risk of recurrence of the disease. Because decitabine has hardly any side effects, it will likely have little impact on the occurrence of Graft Versus Host Disease. The investigators are looking for a pre-treatment for transplantation which reduces the chance of recurrence of the disease without involving severe damage to normal tissues.