The hypereosinophilic syndrome groups together patients with idiopathic eosinophilia and diffuse organ infiltration with eosinophils. It appears to be a continuum of disease from the asymptomatic patient with skin and heart disease at one end to eosinophilic leukaemia at the other. A case is described of a young woman who presented with asthma, mononeuropathy multiplex and eosinophilia and subsequently developed eosinophilic gastroenteritis, vasculitis and probable myocarditis. The response to prednisolone has been pleasing.