Myelodysplastic syndromes (MDS) are the most frequent myeloid neoplasms in Western Countries.They mainly affect patients aged 65 years or older. This is a very heterogenous group of diseases, which prognosis is evaluated with International Prognosis Scoring System. High risk MDS present with high frequency of transformation into acute myeloid leukemia. Treatment of high risk MDS often is based on hypomethylating agents, such as 5'-azacytidine (Azacytidine), with a complete response in approximativel 20% of cases..