Patients with elevated eosinophil counts that do not have a known cause for this alteration, such as a parasitic infection or allergic disorder, were classified as having a hypereosinophilic syndrome of unknown cause It is now understood that some have a leukemic basis for the eosinophil increase Thus, some patients with high blood eosinophil counts have eosinophilic leukemia The chronic form of eosinophilic leukemia is uncommon The acute form is rare