We report a case of hypereosinophilic syndrome associated with acute lymphoblastic leukemia (L1 type, FAB classification) which showed an abnormal karyotype. An 8-year-old boy was admitted to our hospital with complaints of fever and cough that had persisted for 2 weeks. Peripheral blood examination revealed remarkable eosinophilia (120,000/mm3) and a few lymphoblasts. Bone marrow examination also revealed many mature eosinophils and 20% lymphoblasts that were PAS and peroxidase negative. A direct chromosome analysis of the bone marrow cells demonstrated that 12.5% of the spontaneously dividing cells had an abnormal karyotype of 46XY, t(5;14) (q31;q32). The chest radiogram showed interstitial pneumonia-like densities, and the ECG had the pattern of a right bundle branch block. The therapy consisted of prednisolone, high dose of methylprednisolone, cyclophosphamide, and vincristine. This treatment failed to reduce the eosinophil count. On the 4th day after admission, the patient developed severe dyspnea, complete A-V block, and died. At postmortem, dense infiltrations of eosinophils in various stages of maturation were noted in lungs and liver.