Therapeutic options for patients with relapse of MDS or high risk AML after allogeneic stem cell transplantation are limited. We here present the case of a 64-year-old female patient with MDS, who received peripheral blood stem cells from her HLA-identical brother after a non-myeloablative conditioning regimen. Two months after allogeneic transplantation she suffered from a relapse, now fulfilling WHO criteria for AML with a bone marrow blast count of 91%. We then decided to treat her with azacitidine, a DNA methyltransferase inhibitor with proven antileukemic activity. The patient achieved a complete haematological response after two cycles and full donor chimerism after a single dose of donor lymphocytes. We postulate that azacitidine acts through a direct reduction of malignant cells and may in addition augment the immunologic effects of donor lymphocyte infusions.