Statistical Approach to Chronic Lymphocytic Leukemia and Acute Leukemia

To the Editor.— The review article by Zarrabi et al entitled "Chronic Lymphocytic Leukemia Terminating in Acute Leukemia," 1 questions the origin of the acute leukemic termination of chronic lymphocytic leukemia (CLL). The issue raised is whether the occurrence of acute leukemia (AL) with CLL is a second malignancy or a "blast crisis (transformation)," which occurs as a natural evolution of CLL. We would like to demonstrate that the number of cases with CLL and AL reported, and the number predicted, could occur by chance. Since 1936, 2 19 cases of CLL and an associated AL have been reported 1 in the American medical literature. Demographic and morphologic characteristics of the patients with AL have not been consistently reported. The average death rate in 1967 for all ages and sexes was 1.6 per 100,000 for CLL, and 2.4 per 100,000 for AL. 3 The average annual population in the United