The karyotypes in six patients with Ph-positive chronic myelogenous leukemia (CML) were investigated during the lymphoid crisis associated with high levels of terminal deoxynucleotidyl transferase (TdT) and/or the common acute lymphoblastic antigen (CALLA). Five of the six patients had only the Ph chromosome, with no other karyotypic abnormalities. The remaining one patient had a hypodiploid karyotype: 44,XY, -1, + der(1;?)(p22;?), -3, -4, -6, -7, -8, -9, +22q-, + mar1, + mar2, + mar3. In four patients with lymphoid crisis expressing TdT and CALLA, the response to treatment with vincristine (VCR) and prednisolone (PRD) was satisfactory, except for the one patient whose karyotype was hypodiploid. A discussion is presented as to whether or not there is a correlation between the karyotypic changes, using banding methods, and TdT expression in patients whose blast cells were categorized morphologically as lymphoblastic at the onset of the blastic phase of CML. Sequential chromosome examinations during the chronic and blastic phases of CML were also performed in this study.