T-cell acute lymphoblastic leukemia/lymphoma (T-ALL/LBL) is a neoplasm of lymphoblasts that is usually TdT-positive and variably expresses CD1a, CD2, cytoplasmic CD3 (cytoCD3), CD4, CD5, CD7, and CD8. Acute undifferentiated leukemia (AUL) is a rare entity and is defined by the expression of not more than one membrane marker for any given lineage. Blasts express HLA-DR, CD34, and/or CD38 and may be positive for terminal deoxynucleotidyl transferase (TdT). Mixed-phenotype acute leukemia (MPAL) is a hematologic malignancy with immunophenotypic co-expression of markers from at least two cell lineages. In this case series of acute leukemia, we emphasize on key diagnostic or biological points of acute leukemia of ambiguous lineage versus T-ALL and provide a relevant literature review. A diagnosis of AUL can be debatable for our cases; however, the positivity of CD5 and CD7 (bright) ruled out AUL. Although cyCD3 is a T-cell lineage-specific marker of T-ALL, its negativity does not always rule out the diagnosis as several cases have been reported with a negative cyCD3, as reported in our cases.