Eosinophils have been associated with endomyocardial fibrosis. However, their effect on the myocardium and the reactions they elicit have not been emphasized. We describe four patients with extensive myocardial eosinophilic infiltration with and without endomyocardial fibrosis. Patients with a short duration of disease had eosinophilic infiltrates, myocardial eosinophilic abscesses containing granular eosinophilic material, and/or Charcot-Leyden crystals with and without profuse histiocytic and giant cell reaction with granuloma formation. In the early phase, the granular eosinophilic material can be visualized by the Luna stain and/or by electron microscopic examination. In patients with a long duration of symptoms, well-developed granulomas and profuse fibrosis are seen and eosinophilic material may not be visualized. Toxic eosinophil granular protein may be involved in the development of these lesions.