A 19-year-old man with Ph-positive chronic granulocytic leukemia developed lymphoblastic transformation. Cytogenetic evolution was observed, with an abnormal clone showing i(17q) together with the t(9;22). Chronic phase of the chronic granulocytic leukemia were re-established with systemic chemotherapy, which also led to disappearance of the clone with i(17q). However, the acute lymphoblastic leukemia relapsed after 6 weeks, with the emergence of a phenotypically and genetically identical but cytogenetically distinctive clone. Our findings suggest that cytogenetic evolution in transformed chronic granulocytic leukemia reflects only the instability of the blastic clones, and may not determine its phenotypic differentiation.