Ataxia-telangiectasia (AT) is a hereditary disorder characterized by progressive neurological dysfunction, oculocutaneous telangiectasia, immunodeficiency, cancer susceptibility, and radiation sensitivity. Pediatric patients may develop acute lymphoblastic leukemia (ALL). However development of ALL in an adult patient with AT is a rare occurrence. Here we report such a patient who presented with hyperleukocytosis and were treated with leukapheresis. A 25 years old male patient, who were diagnosed with AT and mental retardation, was admitted to the emergency department due to fatigue, nausea and headache. On admission he had a moderate general condition and was fully cooperated. His white blood cell (WBC) count were 466 x 10(9)/l. Blastic cells were observed in peripheral blood smear. Flow cytometry (FC) of peripheral blood showed T-ALL Two sessions of large volume leukapheresis was performed. Symptoms due to hyperleukocytosis markedly improved after leukapheresis. Patients with AT should be closely monitored due to risk of malignancy. Leukapheresis may improve the prognosis of high risk ALL patients presenting with hyperleukocytosis. (C) 2013 Elsevier Ltd. All rights reserved.