This report presents a case of common acute lymphoblastic leukaemia-lymphoma expressing low molecular weight cytokeratin but no leukocyte common antigen (CD45) in a 57-year-old man. The unusual morphology and clinical course together with the aberrant immunohistochemical results suggested a diagnosis of undifferentiated carcinoma. A detailed immunohistochemistry study on frozen and paraffin sections and molecular analysis prevented a diagnostic mistake.