PURPOSE: Hyperleukocytosis secondary to acute leukemia is a medical emergency. Intracranial hemorrhage often leads to death in this setting. Early efforts to prevent this serious complication have included emergent cranial irradiation, with its associated morbidity when used in the young child. Currently, exchange transfusion and/or leukapheresis are employed to acutely lower the peripheral leukocyte count. PATIENTS AND METHODS: We report three infants with acute leukemia and hyperleukocytosis in whom intravenous hydration, alkalinization, and allopurinol therapy alone produced rapid and dramatic decreases in the peripheral leukocyte count. RESULTS: The maximal decrease in leukocyte count averaged 88% within 70 h of starting conservative management. A fall in leukocyte count to < 100 x 10(9)/L was noted at an average of 15 h following hospitalization. No patient developed complications. CONCLUSIONS: When comparing this approach to exchange transfusion and leukapheresis we find it to be both safe and effective. Children with hyperleukocytosis in association with acute lymphocytic leukemia who present without life-threatening complications of an extremely high leukocyte count can be safely and effectively managed with intravenous hydration, alkalinization, and allopurinol therapy.