Background: Chronic myeloid leukemia (CML) is usually diagnosed in the chronic phase. An extramedullary blast crisis mimicking a T-cell lymphoma is a rare finding. Case Report: A 35-year-old man presented with multiple lymphadenopathy 2 months after diagnosis of Philadelphia chromosome (Ph)-positive CML in the chronic phase. Cervical lymph node biopsy later indicated an extramedullary blast crisis resembling T-cell lymphoblastic lymphoma (TLBL). The tumor mass was composed of primitive lymphoid cells expressing terminal deoxynucleotidyl transferase (TdT), CD3, CD43, CD5, CD99, and Bcl-2. Although the pathological diagnosis, confirmed by 2 independent pathological centers, was more typical of TLBL, fluorescence in situ hybridization (FISH) analysis showed the bcr-abl fusion gene within the blastic tumor cells. The FISH finding confirmed that the mass represented an extramedullary, immature blastic transformation of CML rather than a de novo T-cell lymphoma. Conclusion: The diagnosis of de novo TLBL should be suspected before excluding the extramedullary blast phase of CML.