BACKGROUND: Anaplastic large cell lymphoma (ALCL) is a rare type of peripheral T-cell lymphoma, accounting for a small percentage of adult and childhood lymphomas. The scarcity of reports on relapsed or refractory ALCL in children, especially with complex karyotypes, highlights the need for detailed case studies. METHODS: We report the case of a 9-year-old girl diagnosed with ALK-positive anaplastic large cell lymphoma, complicated by complex karyotype abnormalities and bone marrow involvement. The patient underwent multiple chemotherapy regimens, including the P regimen, AV1, BV1, AV2, BV2, and AV3 protocols, followed by continuous monitoring through bone marrow biopsies and PET/CT imaging. Despite initial treatment efforts, the disease relapsed, and the patient's condition deteriorated. RESULTS: Initial treatment with chemotherapy led to a transient reduction in disease activity, as evidenced by bone marrow biopsies showing no malignancy. However, PET/CT scans revealed persistent metabolic activity in multiple skeletal sites, indicating incomplete remission. The patient's condition further relapsed with significant disease progression, culminating in sepsis, heart failure, and active bleeding, which were unresponsive to treatment. CONCLUSIONS: This case underscores the aggressive nature of pediatric ALCL with complex karyotypes and highlights the challenges associated with its treatment. Despite intensive chemotherapy, the disease exhibited rapid relapse and resistance, ultimately leading to a fatal outcome. This report contributes to the limited literature on pediatric ALCL, particularly in cases with complex cytogenetic profiles, and emphasizes the need for novel thera-peutic approaches and early intervention strategies.