PURPOSE: Hyperleukocytosis combined with acute lymphoblastic leukemia (ALL) is associated with early morbidity and mortality because leukostasis interferes with the blood flow by leukocytes within the microcirculation and can result in tissue hypoxia, vascular damage, organ dysfunction, and hemorrhage. But the incidence and nature of complications associated with hyperleukocytosis at the presentation of childhood ALL are not well studied. METHODS: This study was retrospectively performed to review the presenting characteristics, early complications and outcomes in all children treated with newly diagnosed ALL at Pusan National University Hospital between January 1994 and December 2004. Of the total of 100 patients, 17 (17%) children had hyperleukocytosis (leukocyte count >100,000/uL) at diagnosis. RESULTS: T cell phenotype, splenomegaly, thrombocytopenia and hyperuricemia were more frequent in children of ALL with hyperleukocytosis. Early complications during remission induction therapy included neurologic events in 5 (33.3%), respiratory events in 5 (33.3%), renal dysfunctions in 8 (53.3%) of 15 patients with hyperleukocytosis. In 5 (33.3%) of 15 patients with hyperleukocytosis, intravenous hydration, urinary alkalinization and oral allopurinol only resulted in a dramatic drop of the leukocyte count. Complete remission rate was 84.6% and the relapse rate was 23.1% in patients with hyperleukocytosis. 4-year event free survival (EFS) and 4-year overall survival (OS) were 72.2%, 73.3% for the patients with hyperleukocytosis. The patients with leukocyte counts of 100,000~200,000/uL had a better OS than those with leukocyte counts greater than 200,000/ uL (87.5% vs 57.1%). CONCLUSION: The overall survival of ALL with hyperleukocytosis may be poor probably due to early complications related to leukostasis and high relapse rate, especially in patients with extreme hyperleukocytosis. Hydration and alkalinization resulted in a dramatic drop of the leukocyte count in some cases of hyperleukocytosis.