The t(9;22)(q34;q11) translocation between bcr and abl genes plays a pivotal role in the pathogenesis and diagnosis of chronic myelogenous leukemia (CML). Fluorescence in situ hybridization (FISH) using specific DNA probes provides a useful and accurate way for the detection of bcr/abl fusion gene in single cell. Here, we report an unusual case of a patient with no prior hematologic disease who initially manifested lymphadenopathy. The lymph node findings were suspicious for T-lineage lymphoblastic lymphoma, however, his blood and bone marrow at that time were in chronic phase of CML. This presented difficulty for accurate discrimination between CML blast crisis (BC) and non-Hodgkin's lymphomas (NHLs). To discern where the extramedullary nodal malignancy originated from, we cytologically analyzed lymph node biopsies and bone marrow with FISH to detect bcr/abl fusion signals. Together with the morphology, immunohistochemistry, cytogenetics as well as molecular analysis, the patient was diagnosed as extramedullary T-lymphoid BC of Ph+ CML. In conclusion, this case is unusual at three levels: first, extramedullary nodal BC as a presenting manifestation of CML is rare and the blasts are of precursor T lymphoblastic lineage, rather than the more common B-cell lineage; second, this case suggests that extramedullary lymphoid nodal BC of CML can exist independently without the bone marrow developing into BC; and third, FISH analysis on the single neoplastic cell is an accurate way to confirm that the neoplasm is either extramedullary localized blasts of CML or genetically distinct neoplasm. Diagn. Cytopathol. 2013. (c) 2011 Wiley Periodicals, Inc.