There is no standard therapy for elderly patients with high-risk myelodysplastic syndrome (MDS). The treatment options of low-dose Ara-C and haematopoietic growth factors are disappointing in regard to response rate or response duration. We tested the treatment with a 72-h continuous infusion of low-dose 5-Aza-2'-deoxycytidine (DAC) in a group of 29 elderly patients with high-risk MDS. In 15 patients (54%) we observed a response. Eight complete responses were reached, even among patients with bad prognostic cytogenetic findings. The actuarial median survival from the start of the therapy was 46 weeks. The only (and major) toxicity was myelosuppression, leading to a prolonged cytopenic period and thus leading to five toxic deaths (17%) in this high-risk patient group. We conclude that DAC is an effective drug in the treatment of MDS patients and that it probably works via its cytotoxic activity. Myelotoxicity is its major adverse effect.