The identification of the FIP1L1-PDGFRA fusion gene provides a molecular explanation for the pathogenesis of approximately half of the patients with the hypereosinophilic syndrome (HES). A diagnostic test to identify FIP1L1-PDGFRA positive HES cases (subsequently reclassified as chronic eosinophilic leukemia, CEL) is now available. FIP1L1-PDGFR alpha is a novel therapeutic target of the kinase inhibitor imatinib (Glivec, Novartis), which provides the basis for the treatment of these patients with this drug. FIP1L1-PDGFRA positive CEL patients respond very well to imatinib therapy, some of which are remarkable responses with normalization of the blood counts within 2 weeks after start of the therapy. Imatinib is well tolerated with minimal side effects, and most CEL patients respond to low doses of imatinib (100 mg/day), being important for lowering both the cost of therapy and drug related toxicity. All imatinib treated FIP1L1-PDGFRA positive CEL patients achieve hematological and cytogenetic remission, and the majority of patients also achieve a molecular remission with the fusion gene no longer detectable in blood, even by the most sensitive PCR techniques.