A wide range of disorders give rise to eosinophil counts greater than 1.5 X 10(9)/l (hypereosinophilia) and cardiac injury. The best known of these is eosinophilic endomyocardial disease (Löffler's endomyocardial fibrosis), which occurs as a major complication of the idiopathic hypereosinophilic syndrome. Here the heart damage appears to be a direct result of tissue injury produced by toxic eosinophil granule proteins within the heart. However, it is not known what causes the eosinophilia in these patients, why the eosinophils degranulate, or why the endocardium is especially susceptible to this type of injury. A number of parasitic infections may give rise to eosinophilic myocarditis. This is usually the result of the presence of the parasites within the myocardium where they die within inflammatory lesions, which may be extensive. Occasionally, drug reactions and rejection of a transplanted heart may produce eosinophilic myocarditis. Allergic granulomatosis and vasculitis (the Churg-Strauss syndrome), which gives rise to granulomas involving the myocardium, and eosinophilic (hypersensitivity) myocarditis usually respond rapidly to treatment with steroids. However, diffuse myocardial involvement may lead to heart failure, and some of these patients may later develop dilated cardiomyopathy. It is concluded that the heart may be affected by a variety of diseases in which eosinophils are a prominent component in the inflammatory cell infiltrates. Eosinophils themselves may contribute to some of the myocardial cell injury which occurs in these diseases, and attempts to limit this with steroids may be worthwhile in some patients.