We report a 4.7 kg infant who received a therapeutic leukapheresis as an immediate treatment for acute lymphoblastic leukemia with severe hyperleukocytosis. By decreasing the number of circulating white blood cells, therapeutic leukapheresis helps prevent the risks of hyperviscosity and cerebrovascular and pulmonary leukostasis. In addition, it potentially reduces metabolic and renal complications associated with rapid cell lysis when applied before chemotherapy. This six-week-old female presented with vomiting for 15 days. Initial WBC count was 1,532,800/μL. After placement of 4 french two-lumen central venous catheter in both femoral vein, the CS 3000 plus was primed with 250 mL of paternal whole blood mixed with 150 mL of normal saline. After therapeutic leukapheresis, the CBC showed WBC count of 560,000/μL. Our successful experience in performing this procedure suggests that therapeutic leukapheresis be a feasible treatment even for very young infants with hyperleukocytosis.