Additional cytogenetic changes in blastic phase chronic myeloid leukemia (CML) occur in 60-80% of cases. A 70 year-old patient with chronic myeloid leukemia (CML) was referred for cytogenetic investigation at diagnosis and for periodic follow-up during evolution. Cytogenetic studies were performed on bone marrow chromosomes, by GTG banding. At diagnosis, t(9;22)(q11;q34) has been the sole anomaly. The progression to blastic crisis occurred 2 years after diagnosis and was preceded by aberrant basophilia in bone marrow only. The cytogenetic investigation revealed an additional abnormality: loss of chromosome Y. A short duration remission was achieved with Glivec therapy. Subsequently the patient reentered blastic phase with atypical eosinophilia and basophilia, both in peripheral blood and bone marrow. A high percentage of hybrid granulocytes (eosinophils and basophils) with lineage infidelity markers was also observed. Cytogenetic reevaluation showed besides t(9;22)(q34;q11) and loss of chromosome Y, isochromosome i(17q), addition add(12q). The additional chromosomal abnormalities might be related to the morphological abnormalities of the basophilic and eosinophilic lineage, in our case. The case evolution was particularly unfavorable with poor treatment outcome. Acknowledgements: National Program for Research of Excelence CEEX MIII, Project no 15/2006; National Program VIASAN Project no 242; CNCSIS Program.