We report on a case of a 30-year-old male with acute B-lymphoblastic leukemia (B-ALL) with immunophenotype CD19(+), CD22(+), CD20(+), CD10(+), with aberrant expression of CD13 and CD117, and IgH gene rearrangements. Three months after treatment with GMALL-2003 and Ida/FLAG protocols bone marrow showed predominance of blasts with myeloid morphology and phenotype MPO(+), CD13(+), CD33(+), CD64(+), CD15(+), CD56(+), EVI-1 gene overexpression and lack of IgH rearrangements. The case is the first report of a very early emergence of myeloid leukemia during the induction treatment for B-ALL in an adult patient. Different pathogenetic mechanisms are discussed - clonal evolution or selection, lineage switch or development of a de novo or therapy-induced leukemia.