Thirty-two patients in the blastic phase of Philadelphia chromosome-positive chronic granulocytic leukemia (CGL) were studied in a prospective randomized trial in which vincristine--prednisone (19 patients) was compared with cytosine arabinoside--6-thioguanine (13 patients). Seven remissions (37%), including two complete remissions, were achieved in the vincristine--prednisone group. Three of the five with predominant hypodiploid blast cell lines treated with vincristine--prednisone had complete or partial remissions. Both complete remitters presented with hypodiploidy consisting of 44 chromosomes. Four patients (30%) who were treated with cytosine arabinoside--6-thioguanine responded with one complete remission. The median survival of the responders was 8 mo, as compared to 1--2 mo for the nonresponders. Crossover to the opposite regimen as secondary therapy following refractoriness or resistance resulted in only 3 partial responses out of 21 treated. All three had previously responded to vincristine--prednisone. Of the 32 cases, 14 had an elective splenectomy during the chronic phase of the disease. Prior splenectomy did not influence the response to chemotherapy, as all three complete remitters occurred in the nonsplenectomized group. Similarly, survival in the blastic phase was not affected by prior splenectomy.