Patients with AML or myelodysplastic syndrome who relapse after allo-SCT have a poor prognosis. In the search for novel treatment strategies for these patients, we conducted a multicenter retrospective analysis and identified 22 patients treated with the DNA-methylation inhibitor 5-azacytidine (5-Aza). Patients received a median number of two cycles 5-Aza (range 1-8) at a dose of 100 mg/m(2) over 5 days following relapse. Eighteen patients (82%) also received a median number of two donor lymphocyte infusions (DLI, range 1-5). Sixteen patients (72%) responded to 5-Aza treatment and five patients (23%) achieved a CR. 5-Aza-induced CR lasted for 433 days (median, range 114-769). Median survival and the estimated 2-year survival rate were 144 days and 23%, respectively. Acute GVHD after DLI was seen in six patients (33%) and four of these patients developed chronic GVHD of the skin. There were no treatment-related deaths. Patients who achieved halving of leukocyte counts after the first 5-Aza cycle had a superior median survival of 802 days compared with 135 days (P=0.0025) in all other patients. On univariate analysis, the achievement of this halving of leukocyte counts was identified as a significant predictor of survival.