Myeloproliferative/myelodysplastic syndromes are rare diseases that include a proliferative component, mainly on the white cells and platelets, and a dysplastic component that accounts for one or several cytopenias. The most frequent of these diseases in chronic myelo-monocytic leukemia, a disease of elderly people that has long been associated with myelodysplastic syndromes in biological studies as well as in clinical trials. The recent identification of a number of genetic mutations in the leukemic clone, including frequent mutations in TET2, ASXL1 and RUNX1, less frequent mutations in NRAS, KRAS and C-CBL, and rare mutations in JAK2, FLT3, IDH1, IDH2, and EZHR2 may improve our understanding of the pathogenesis of this disease. Patient care depends on the disease risk, especially the percentage of blast cells in the bone marrow, the age and the performance status. Supportive care is required in all patients. In high risk patients, the only curative therapeutic is allogeneic hematopoietic stem cell transplantation, which is rarely feasible due to the age of the patients and the absence of donor. Demethylating agents such as azacitidine and decitabine are currently the most efficient drugs. The prognosis remains poor, with a median survival lower than 24 months.