We assessed the frequency and outcome of complications associated with hyperleukocytosis in children who had received remission induction therapy for either acute lymphoblastic leukemia (ALL) or acute nonlymphoblastic leukemia (ANLL). Among 234 consecutive patients with a leukocyte count of 100 X 10(9)/L or greater at diagnosis, the frequency of early death was significantly higher in those with ANLL (23% v 5% for patients with ALL, P less than .001). The risk of early death increased with increasing leukocyte count, especially when it exceeded 300 X 10(9)/L in patients with ANLL (P less than .001). Intracerebral hemorrhage occurred in eight (11%) of the ANLL patients and accounted for six of the 17 early deaths; in all but two instances, the complication was associated with coagulopathy. Respiratory failure, presumably from pulmonary leukostasis, resulted in six other early deaths in ANLL patients. By contrast, intracerebral hemorrhage occurred in only two of the ALL patients (1.2%); both had normal coagulation studies, but leukocyte counts greater than 400 X 10(9)/L. Severe metabolic derangements from blast cell lysis accounted for three of the eight early deaths among patients with ALL. Leukapheresis or exchange transfusion effectively lowered the leukocyte counts of all 15 patients who received the procedures. Either method may be preferable to emergency cranial irradiation for preventing the complications of hyperleukocytosis in children with acute leukemia.