According to the Surveillance, Epidemiology and End Results database, the estimated morbidity is between 0.036 and 6.3/100,000,[1] while the mortality is about 9.3%.[2] Usually, HES was thought to affect mostly males, with a male:female sex ratio estimated at 9:1, especially in FIP1L1-PDGFRα (F/P)-associated HES who are almost exclusively male. This evidence concerns the gene PDGFRA and hypereosinophilic syndrome.