Although lineage fidelity is generally maintained in acute leukemia, a proportion of cases have been reported in which leukemic cells in appropriately express features normally confined to separate lineages. Numerous terms have been used to designate these leukemias including hybrid, biphenotypic, bilineal, mixed, however, a standard terminology is still lacking. In this work we are aiming at proper characterization of hybrid acute leukemia cases in a trial to classify them according to the various phenotypic patterns and study the clinical relevance of the different finding.