Idiopathic hypereosinophilic syndrome (IHES) is a heterogeneous group of disorders characterized by multisystem dysfunction and persistent, extreme eosinophilia of unknown cause. We describe a 9-1/2-year-old boy whose course included several unusual clinical features and terminated 2 years after diagnosis in acute lymphoblastic leukemia (ALL). Serial studies suggest that leukemia was not present earlier in his course. We speculate that this child may have had an evolving lymphoproliferative syndrome with a terminal blast crisis to which the eosinophilia was a nonmalignant leukemoid reaction.