We describe an uncommon case of acute leukemia in which leukemic blasts expressed myeloid antigens and cyCD79alpha molecule. In this 49-year old male patient, two distinct blast populations were detected in peripheral blood and bone marrow samples: one of small size resembling lymphoblasts and another with pink cytoplasmic granules resembling myeloblasts. Cytochemical reaction for myeloperoxidase was negative in both cell types. Conventional cytogenetic analysis showed a normal karyotype (46 XY) in all metaphases studied, while gene rearrangement analysis by seminested PCR of the immunoglobulin heavy chain (Ig-H) and T-cell-gamma chain (TCR-gamma) receptor, showed a germline configuration of the TCR and clonal rearrangement of Ig-H chain genes. Multicolour cytofluorimetric analysis showed that bone marrow and peripheral blood blasts expressed CD19, CD79alpha bright, CD22 and terminal deoxynucleotidyl transferase (TdT) as lymphoid markers, CD13, CD117, CD15 as myeloid markers, CD34, HLA-DR as stem cell markers. CD33 myeloid antigen was expressed by 50% of the blastic population. No differences in the immunophenotypic profile were detected in the two blast populations which were identified by morphology. According to EGIL (European Group of Immunological Classification of Leukemias) and WHO (World Health Organization) criteria, a diagnosis of biphenotypic acute leukemia (BAL) was made. The patient was treated with AML induction therapy followed by autologous stem cell transplantation, but relapse free survival was 6 months. The patient died a few weeks later due to unresponsiveness to salvage chemotherapy regimens. We conclude that patients with BAL should have a risk stratification with treatment tailored to their immunophenotype and gene rearrangement profiles.