Lenalidomide has been successfully used in MDS patients with del5q, irrespective of additional cytogenetic abnormalities. Furthermore, in vitro studies have demonstrated also impressive anti-proliferative effects of the compound in cell lines harbouring a monosomy 5. Therefore, it seems to be a promising compound in preventing relapse of high-risk MDS or AML patients with chromosomal abnormalities involving del5q or -5 after allogeneic HSCT. Due to its immunomodulatory action it might also be able to enhance T - or NK cell mediated graft versus leukemia effects. Nevertheless, it is unknown whether lenalidomide could modulate or enhance clinical graft versus host disease.