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.