Acute lymphoblastic leukemia with eosinophilia is a rare but distinctive clinical entity. The eosinophilia in these patients can present before, concomitantly, or after the diagnosis of leukemia. Patients with this syndrome often suffer from the cardiovascular complications of severe eosinophilia, suffering excess morbidity and mortality as a result of their eosinophilia. Treatment of the eosinophilia in this syndrome consists of administration of induction chemotherapy, followed by prednisone and hydroxyurea if required for persistent eosinophilia. Eosinophilia often resolves with remission of leukemia, only to return at the time of relapse in a high percentage of cases. Patients with this syndrome characteristically have cytogenetic abnormalities involving the long arms of chromosomes 5 and 14. These cytogenetic abnormalities are not commonly seen in acute lymphoblastic leukemia and suggest that this syndrome may have a distinct pathophysiology and etiology. The affected region on chromosome 5 contains genes that control hematopoiesis, including eosinophilopoiesis. Further investigations into these cytogenetic abnormalities may provide insight into the etiology of the leukemia and eosinophilia characteristic of this syndrome.