The effect of splenectomy during the blastic phase (BP) in Philadelphia (Ph1) positive chronic myeloid leukemia (CML) was correlated with the chromosome findings during that phase of the disease. Fifty-three patients were divided into six groups on the basis of the chromosome findings and splenectomy; i.e., nonsplenectomized and splenectomized patients with only a Ph1 clone (PP/S- and PP/S+); nonsplenectomized and splenectomized patients with a clone containing only a Ph1 and another clone with karyotypic changes in addition to the Ph1 (AP/S- and AP/S+), and nonsplenectomized and splenectomized patients with only abnormal clone(s) containing karyotypic abnormalities in addition to the Ph1 (AA/S- and AA/S+). The percentage of patients with a good therapeutic response in each group was higher in splenectomized than nonsplenectomized patients. The median survival after the onset of BP was 171 days (5.7 months) in PP/S-, 167 days (5.6 months) in PP/S+, 136 days (4.5 months) in AP/S-, 183 days (6.1 months) in AP/S+, 23 days (0.8 months) in AA/S-, and 155 (5.2 months) in AA/S+. Statistically, there was a significant difference only between AA/S- and the other groups. These findings may indicate that splenectomy during the blastic phase could contribute to an improved tolerance to chemotherapy in each group and prolong the survival after the onset of BP in AA patients.