Ten patients under 20 yr of age with the usual (adult) type of chronic myelogenous leukemia (CML) were seen at the University of Rochester Medical Center from 1970 to 1982. The mean white cell count in these 10 patients at presentation was 360,000/microliter, as compared to a mean of 137,000/microliter in 80 CML patients over 20 yr of age seen during the same time interval (p less than 0.02). Analyses of all 90 cases revealed a significant decrease in the average leukocyte count at presentation with increasing age. The childhood cases also had a significantly higher proportion of blood blasts, promyelocytes, and myelocytes than did the adult subjects (p less than 0.01). Signs of leukostasis were present in 12% of adult cases as compared to 60% of the 10 childhood cases, and in these 6 subjects, the mean white cell count was 510,000/microliter. In these 6 patients, leukapheresis and/or chemical therapy was initiated rapidly, and this was followed by complete resolution of the clinical signs of leukostasis. A review of the literature from 1960 to 1982 identified 61 childhood cases that were reported with the usual type of CML. In this group, the frequency of hyperleukocytosis and the distribution of white cell counts corresponded very closely to the 10 cases studied at the University of Rochester. Thus, the usual type of CML presenting in childhood differs from that of adults in that hyperleukocytosis, blood granulocyte immaturity, and leukostatic central nervous system, retinal, and respiratory signs are significantly more common and extreme and merit rapid cytoreductive treatment.