Median survival figures are essentially worthless for evaluation of the effectiveness of therapy in different series of patients with chronic myelocytic leukemia. Such data are heavily influenced by the characteristics of the initial patient mix, which play the major role in determining mortality during the first year of treatment. The effects of differing initial proportions of poor-risk patients are dissipated within two years, however, and the annual mortality rates thereafter reflect the influence of antileukemic therapy. Comparison of these rates, from very diverse series of patients, yielded quite consistent results and indicated superiority of effective chemotherapy over irradiation or inferior drug schedules.