Two hundred forty-two patients with Philadelphia chromosome-positive chronic myelogenous leukemia in blast crisis were reviewed to identify significant biologic and prognostic associations. Twenty percent of patients had lymphoid blast crisis. Clonal evolution was present in 60 percent of patients at blast crisis and involved most frequently the development of a double Philadelphia chromosome, trisomy 8, or isochromosome 17. The overall median survival from blast crisis was 18 weeks. Patient characteristics demonstrated to have significant association with short survival were: anemia; thrombocytopenia; myeloid or undifferentiated blast cell morphology; clonal evolution involving the presence of a double Philadelphia chromosome, trisomy 8, or isochromosome 17; and low marrow blast percentage. Of 195 patients who received therapy for blast crisis, complete remission was achieved in 44 (23 percent) patients, and 24 (13 percent) patients had a partial remission or hematologic improvement. Lower complete remission rates were associated with old age, thrombocytopenia, myeloid or undifferentiated blast cell morphology, clonal evolution--especially isochromosome 17 and trisomy 8--and long interval from diagnosis to onset of blast crisis. A multivariate analysis identified two characteristics to have independent prognostic importance for both survival and remission: platelet counts and blast cell morphology. In addition, clonal evolution had additive prognostic value for survival (double Philadelphia chromosome) and for response (isochromosome 17). The beneficial association of therapy with survival was demonstrated by the significantly longer median survival of patients treated since 1981 compared with those treated earlier, even after accounting for the pretreatment prognostic factors, and by the significant improvement in survival of patients achieving remission using the "landmark" analysis technique.