The case was attended by our team at Santa Casa de São José dos Campos, a teaching hospital with Accreditation Level 3 from National Accreditation Organization, classified as structuring hospital by the government of the State of São Paulo, which serves patients of the public health system (SUS) and is a reference in complex care including onco-hematological diseases. A 65-year-old female patient was admitted with complaints of asthenia, weakness in the lower limbs, fever, night sweats and weight loss of 14 kg over a period of 10 months. Initial workups showed hemoglobin 7.7 g/dL, MCV 82.7fL, RDW 19.4%, white blood cells 13.6x109/L, segmented neutrophils 1.1x109/L, blasts 7.9x109/L, presence of erythroblasts and platelets 23x109/L. Renal and hepatic functions were unchanged and serologic tests negative. Myelogram evidenced substitution of normal myeloid elements by 66% of moderate size blasts with high nucleo-cytoplasm ratio, basophilic cytoplasm some with microvacuolization, loosely attached chromatin and evidently surplusing nucleoles. Megakaryocyte were not observed. The cytological picture was suggestive of acute leukemia. Immunophenotypic analysis detected two anomalous and distinct cell populations. Population one with myeloid blasts (32.1% of total events acquired), CD7, CD13, CD33, CD34, CD38++, CD56, CD71+, CD81+, CD117, CD123+, HLA-DR and MPO (10%) positive markers. The second population was characterized by blasts co-expressing B and T lymphoid antigens (22.1% of total acquired events), CD2, cytoplasmic CD3, CD7, CD13, CD19, CD22, CD33, CD34, CD38+, CD56, CD71+, CD79a, CD81+, D117, CD123+ and HLA-DR positive markers. The immunophenotypic findings were compatible with Acute Leukemia of Mixed B, T and Myeloid phenotype [Mixed-phenotype acute leukemia (MPAL)]. Complex and compound karyotype showing monosomy of chromosome X, translocation between the long arms of chromosomes 2 and 5, 7 and 14, translocation between the short arm of chromosome 4 and long arm of chromosome 7, partial deletion of the long arm of chromosome 5, monosomy of chromosome 5, short arm isochromosome of chromosome 6, interstitial deletion of the long arms of chromosomes 7 and 9, monosomy of chromosomes 8, 9, 14, 16, 20 and 21, partial deletion of the long arm of chromosome 14 and trisomy of chromosome 20. Throughout hospitalization, the patient had evident thrombocytopenia (values below 5x109/L) and high platelet transfusion dependence. The regimen of hyperfractionated cyclophosphamide, vincristine, doxorubicin, and dexamethasone (hyper-CVAD) was proposed for induction of remission, however three days after initiation the patient presented acute respiratory failure due to probable alveolar bleeding unresponsive to mechanical ventilation, cardio respiratory arrest and death. MPAL is characterized by blasts with phenotypic markers of two or three hematopoietic lineages that cannot be clearly defined as acute myeloid or lymphoblastic leukemia. Cytogenetic aberrations are usually present and often associated with higher risk somatic mutations. Because it is a rare disease there are no treatments based on multicenter prospective studies. Anecdotal evidences have suggested that induction protocols used in the treatment of acute lymphoblastic leukemia followed by allogeneic bone marrow transplantation result in better outcomes when compared to treatments for acute myeloid leukemia. However, despite efforts to provide initial remission the prognosis remains poor.