Introduction: 5-Azacytidine is a pyrimidine nucleoside analog of cytidine that undergoes incorporation into DNA and blocks DNA methyltransferase leading to hypomethylation and potentially beneficial re-expression of abnormally silenced genes. It is the first agent approved for use in patients with myelodysplastic syndromes (MDSs) based on its improvement in overall survival as monotherapy. Evidence of efficacy in combination with other agents is also accumulating. Areas covered: Key information on mechanisms of action is presented. Development, synthesis, and pharmacokinetics are also outlined. Key safety, tolerability, and efficacy data from clinical trials of 5-azacytidine as monotherapy as well as in combination are also presented. Expert opinion: Our understanding of the molecular basis and pathogenesis of MDS continues to evolve rapidly. 5-Azacytidine has been shown to improve both overall survival and quality of life in patients with high-risk MDS. Currently, the oral route of administration is undergoing evaluation in clinical trials. Used as a monotherapy and also in novel combinations, 5-azacytidine has the potential to further improve the prognosis of some patients with MDS.