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Objectives: To compare the features of the blast phase of chronic myelogenous leukemia (CML) in patients treated with tyrosine kinase inhibitors (TKIs) with those in the pre-TKI era. Methods: Sixty-seven patients with blast phase CML were identified in the Duke Pathology database from 1991 to 2011. The morphology and immunophenotype of blasts were evaluated, along with cytogenetic studies and associated findings in the peripheral blood and bone marrow. Results: In the TKI era, the blasts were more frequently of a type other than the usual myeloid or lymphoid types when compared with the pre-TKI era. Blast phase in TKI-treated patients was associated with a higher peripheral WBC count and a lower blast percentage in the bone marrow. Of the 23 patients with cytogenetic studies during blast phase, additional cytogenetic changes more frequently occurred in patients with an unusual blast type, and some patients showed these changes months before the onset of blast phase. Conclusions: Blast phase CML in TKI- and non-TKI-treated patients differs in the morphology and immunophenotype of blasts, cytogenetic findings, and associated findings in the peripheral blood and bone marrow. [ABSTRACT FROM AUTHOR]