BACKGROUND: The goal was to investigate the clinical characteristics of acute T-lymphoblastic leukemia (T-ALL) transforming into acute myeloid leukemia (AML) and to provide a basis for improved clinical diagnosis and treatment of this condition. METHODS: A retrospective analysis was performed on the clinical data and treatment process of a patient with T-ALL who transitioned to AML after undergoing allogeneic hematopoietic stem cell transplantation. Relevant literature was also reviewed. RESULTS: The patient, a 24-year-old female, initially presented with fatigue. A comprehensive examination confirmed the diagnosis of T-ALL. Following induction chemotherapy, the patient achieved complete remission (CR) as determined by hematologic and cytogenetic assessments. The patient subsequently underwent allogeneic hema-topoietic stem cell transplantation from a sibling donor. Six months post-transplantation, bone marrow cytology indicated a relapse, with morphology and immunophenotype consistent with AML. The patient is currently in the terminal stage of a malignant tumor, with a very poor prognosis. CONCLUSIONS: Lineage switching following acute leukemia relapse is associated with a poor prognosis, necessitating treatment adjustments based on the post-relapse phenotype. Clinically, for recurrent leukemia, comprehensive evaluations-including leukemia immunophenotyping, gene mutation analysis, and cytogenetic and molecular biological examinations-are essential to better guide treatment and assess prognosis.