Mixed phenotype acute leukemia is a rare subtype of leukemia that probably arises from a hematopoietic pluripotent stem cell. The co-expression of two of myeloid, B- or T-lymphoid antigens is the hallmark of this disease. Herein, the case of a 28-year-old female patient is reported who presented with hemoglobin of 5.8 g/dL, white blood cell count of 138 × 109/L and platelet count of 12 × 109/L. The differential count of peripheral blood revealed 96% of blasts. Moreover, the patient presented with lymphadenopathy, splenomegaly and bone marrow infiltration by monocytoid blasts characterized as 7% positivity by Sudan Black cytochemical staining. Immunophenotyping revealed the involvement of blasts of both T- and monocytic lineages. The cytogenetic analysis showed an isolated 17p deletion. Thus, the diagnosis of T-cell/myeloid mixed phenotype acute leukemia was made with two particular rare features, that is, the monocytic differentiation and the 17p deletion as unique cytogenetic abnormalities. The possibility of concomitant expressions of T-cell and monocytic differentiation antigens in the same blast population is hard to explain using the classical model of hematopoiesis. However, recent studies have suggested that myeloid potential persists even when the lineage branches segregate toward B- and T-cells. The role of an isolated 17p deletion in the pathogenesis of this condition is unclear. At present, the patient is in complete remission after an allogeneic stem cell transplantation procedure.