Hyperleukocytosis is defined as peripheral blood leukocyte count exceeding 100,000/mm(3). Acute leukemia is the most common etiology in pediatric practice. Hyperleukocytosis is a medical emergency. The increased blood viscosity, secondary to high white cell count and leukocyte aggregates, results in stasis in the smaller blood vessels. This predisposes to neurological, pulmonary or gastrointestinal complications. In addition, patients are at risk for tumor lysis syndrome due to the increased tumor burden. Initial management includes aggressive hydration, prevention of tumor lysis syndrome, and correction of metabolic abnormalities. A red cell transfusion is not indicated in a hemodynamically stable child, as it adversely affects the blood viscosity. Leukapheresis is the treatment of choice for a very high count, or in patients with symptomatic hyperleukocytosis. The technical expertise required, a relative difficult venous access in younger children, risk of anticoagulation and possible non-availability of the procedure in emergency hours are limitations of leukapheresis. However, it is a rewarding procedure and performed with relative ease in centers that perform the procedure frequently. An exchange transfusion is often a practical option when hyperleukocytosis is complicated with severe anemia. The partial exchange aids in correcting both, without the risk of volume overload or hyperviscosity, which are the limitations of hydration and blood transfusion, respectively. Etiology and management of hyperleukocytosis in relevance to the pediatric emergency room is outlined.