The findings presented indicate that splenectomy during the blastic phase (BP) of Philadelphia (Ph) positive chronic myeloid leukemia (CML) prolonged survival after the onset of the BP in a group of patients who had, in addition to the Ph, only chromosomally abnormal cells in the marrow (AA patients). To evaluate the predictive parameter for splenectomy in the chronic phase (CP) of CML, the cytogenetic data obtained during the CP of the AA group were compared retrospectively with those of cytogenetically defined groups. Only AA patients showed the presence of abnormal clone(s) containing abnormalities such as trisomy 8, i(17q), and a missing Y chromosome several years before the onset of BP. This may indicate that the presence of these chromosome abnormalities during the CP could be utilized as a predictive parameter for splenectomy in the CP of CML.