The year 2008 marks the fifth anniversary since the publication which identified the FIP1L1-PDGFRA fusion gene in patients with idiopathic hypereosinophilia. With the benefit of time, a more comprehensive picture has emerged regarding several characteristics of the fusion, including its incidence, biological features and the clinical profile of patients who carry the molecular rearrangement. A few prospective trials have now better defined the natural history of imatinib-treated FIP1L1-PDGFRA-positive patients, from which some basic conclusions can be drawn: the prognosis is outstanding, acquired resistance is exceedingly rare, but ongoing imatinib treatment is likely required to prevent relapse. The emergence of genetically assigned eosinophilias has led the World Health Organization in 2008 to adopt a semi-molecular classification scheme, with one subcategory named 'myeloid and lymphoid neoplasms with eosinophilia and abnormalities of PDGFRA, PDGFRB or FGFR1.' Molecular rearrangements involving other partner genes, such as ETV6 and JAK2, have also been associated with eosinophilic disorders, and will likely be assimilated into such classifications over time. Despite the molecularly defined eosinophilias comprising a small proportion of cases compared to the aggregate of other subtypes of hypereosinophilia, their recognition is critical because of the availability of highly effective molecularly targeted therapy.