The hypothesis of this study is that lenalidomide can be an effective drug in preventing relapse of MDS and AML patients with chromosomal abnormalities involving monosomy 5 or del5q after allogeneic HSCT. Due to its immunomodulatory action it might also be able to enhance a T - or NK cell mediated graft versus leukemia (GVL) effects. Nevertheless, one has to keep in mind a possible, yet unknown influence on modulation of clinical GVHD.