Hyperleukocytosis, defined as a peripheral leukocyte count ≥ 100×109/L, is seen in 5-20% of newly diagnosed cases of childhood leukemia and is a poor prognostic factor. In this study, we aimed to examine the presenting clinical and laboratory features, complications, and treatment outcome of 47 children with acute lymphoblastic leukemia (ALL) and hyperleukocytosis who were diagnosed and treated in four medical centers of İzmir between January 1990 and January 2001. The median age was 5.0 years (range: 0.1-16.3 years). Median white blood cell count was 495×109/L (range: 107×109/L-794×109/L). Forty-two of 47 patients (90%) had hepatosplenomegaly, 5 (11%) had respiratory distress, 3 (6%) had neurologic symptoms, 3 (6%) had diffuse cervical lymphadenopathy, and 3 (6%) had acute renal failure at admission. Ten of 47 patients (21%) had central nervous system involvement, and 17 (36%) had mediastinal mass. Ten patients (21%) had coagulopathy and 15 patients (32%) had metabolic complications (8 patients had hyperuricemia, 4 had hyperphosphatemia, 2 had hyperuricemia, hyperphosphatemia and hypercalcemia, and 1 had hypocalcemia) before the initiation of therapy. Forty of 47 patients (85%) with hyperleukocytosis were effectively managed with intravenous hydration, alkalinization, and allopurinol therapy. Early death during remission induction therapy occurred in 5 patients (11%) with respiratory distress and sepsis. Kaplan-Meier estimates of event free survival and overall survival were 37.0% and 40.5%, respectively. © Turkish Society of Hematology.