Eosinophilic myocarditis is caused by activation of eosinophilic granulocytes whereby there is a release of eosinophilic granules. Quite a few of the released compounds, especially eosinophilic cationic proteins, have a tissue-damaging effect also in the myocardium. Eosinophilia may be due to hypersensitivity, parasitic infection etc. However, in hypereosinophilic syndromes and Loeffler’s endomyocardial disease, the eosinophilia is“idiopathic.” I believe that a clinical spectrum of the disease exists that is intimately correlated with the number of eosinophils and especially the degree of activation. When more is known about the cardiotropism of eosinophils in the various clinical settings, an important step will have been taken toward both the understanding and the treatment of this disease spectrum.