An increase in the blood eosinophil count may occur in a number of disease states including allergies, parasitic infections, vascular disease and as a reaction to the presence of malignant tumours. This article defines those disorders that are not purely reactive, and describes in detail the diagnosis and features of clonal eosinophilic disorders and the hypereosinophilic syndrome. The clonal disorders that are associated with eosinophilia are discussed, in particular the acute and chronic eosinophilic leukaemias and clonal eosinophilias in association with acute myeloid leukaemia, myeloproliferative disorders and myelodysplastic syndromes. Whether eosinophilia is produced by a clonal or reactive disorder, the end result can often be the same, i.e. end organ damage produced by sustained hypereosinophilia in the presence of eosinophil activation. When no cause for the eosinophilia leading to the end organ damage is found, this disease is termed 'idiopathic hypereosinophilic syndrome'. Its pathogenesis, clinical features and management are discussed with particular reference to the possibility of it being a T-cell-associated disorder.