The preliminary results of a disease-oriented phase I-II study aimed at evaluating the clinical activity of 5-aza-2'-deoxycytidine (Decitabine) in patients affected by advanced myelodysplastic syndromes (MDS) are reported. Two patients affected by refractory anemia with excess of blasts (RAEB) and eight with RAEB in transformation (RAEB-T) were treated with Decitabine at a daily dose of 45 mg/m2, divided into three 4 h infusions for 3 days (six patients) or as continuous infusion of 50 mg/m2 for 3 days (four patients). Treatment with Decitabine resulted in a significant increase in circulating neutrophils, platelets, and hemoglobin with respect to pretreatment values in over 50% of patients. These changes were accompanied by the improvement of the marrow myeloid relative differentiation index (median fivefold increase in the whole group of patients) and of the myeloid to erythroid cell ratio (median twofold increase) in most of the patients. In four out of ten patients a complete normalization of peripheral blood (PB) and bone marrow (BM) picture (complete hematologic response) was obtained. The evaluation of the percentage of CD34-positive BM cells showed a slow but progressive reduction of early leukemic progenitors in most of the patients. A transient slight BM hypoplasia was obtained in less than 50% of patients while a severe marrow aplasia was never observed in our group of MDS patients during treatment with Decitabine. Extra-hematological toxicity was very mild in all the patients. The preliminary results of our study indicate that Decitabine is able to induce trilineage hematological responses in advanced MDS patients along with a stable normalization of the PB and BM picture in some of the subjects. Decitabine appears an active agent in advanced MDS and this deserves careful investigation in this heterogeneous group of disorders.