A case of Philadelphia chromosome (Ph1) positive chronic granulocytic leukemia (CGL) is described in which the patient underwent successful treatment with supralethal chemoradiotherapy and allogeneic bone marrow transplantation (BMT) after transformation to blast crisis. Supraclavicular adenopathy developed 5 months after BMT and biopsy revealed a hematopoietic lymphoid neoplasm with an early T cell phenotype. A concurrent bone marrow was microscopically and cytogenetically normal. A metaphase chromosome preparation could not be obtained from nodal tissue. Lymph node DNA, however, was easily extracted and a rearrangement of BCR identical to that in the bone marrow prior to BMT was demonstrated indicating recurrent CGL rather than a de novo lymphoproliferative process. Appropriate therapy for lymphoid blast crisis resulted in a marked regression of measurable disease. The BCR probe may prove to be a useful tool for the diagnosis of CGL when standard cytogenetic techniques cannot be applied.