Because of advances in our understanding of the hypereosinophilic syndrome (HES) and
the availability of novel therapeutic agents, the original criteria defining these
disorders are becoming increasingly problematic. Here, we discuss shortcomings with
the current definition of HES and recent developments in the classification of these
disorders. Despite significant progress in our understanding of the pathogenesis of
some forms of HES, the current state of knowledge is still insufficient to formulate
a new comprehensive etiologic definition of HESs. Nevertheless, we suggest a new working
definition that overcomes some of the most obvious limitations with the original definition.