A 12-year-old boy was admitted to our hospital because of fever, tachypnea and huge hepatomegaly. The patient showed WCB 191,000/μl consisting of 74% blast cells, a large mediastinal mass and diffuse interstitial infiltrates on chest X-ray. By bone marrow examination he was diagnosed as acute monocytic leukemia (AMoL) (FAB-M5b). Before induction chemotherapy, two exchange transfusions were performed in order to reduce the WBC count and prevent complications: however he developed respiratory failure, which was not improved by mechanical ventilation, and died 3 days after admission, probably due to pulmonary leukostasis.Chromosome analysis revealed an abnormality of del (11) (q21q24), which is known to be associated with AMoL. New therapeutic measures are necessary to improve the prognosis of patients with this type of AMoL.