In the WHO classification atypical chronic myeloid leukemia (CML) has been considered as a new distinct clinical entity included in the category of mixed myeloproliferative/myelodysplastic disorders. Little is known about this uncommon disease, whose incidence is about of one-two cases every 100 cases of Ph-positive CML. We analyzed our series of 55 patients diagnosed as having aCML, with the aim of identifying clinical factors of possible prognostic value on survival and acute transformation.