Hypomethylating drugs are useful and have been approved for the treatment of myelodysplastic syndromes (MDS) and Chronic Myelomonocytic Leukemia (CMML). However, phase 2 and 3 studies that assessed these agents in MDS, have included only a small number of patients with CMML, and there are just a few specific reports on CMML patients. The Azacitidine is actually authorised for the treatment of CMML patients with 10-29% marrow blasts without myeloproliferative disorder, who are not eligible for haematopoietic stem cell transplantation. This hypomethylating agent in MDS is known for causing transient cytopenias, most often occurring during the first 2 cycles. Here we report a case of an atypical delayed and prolonged hematologic toxicity during Azacitidine treatment in a CMML patient; furthermore we also reviewed the literature regarding the efficacy of the drug and the management of hematologic adverse effects, in term of dose adjustments or alternative schedule of administration, in specific CMML setting.