Described herein are 3 adults in whom histologic study of the left ventricular myocardium excised ("LV core") to insert a left ventricular assist device (LVAD) disclosed severe acute myocarditis and the inflammatory cells included numerous eosinophils (eosinophilic myocarditis). Examination of the clinical records disclosed elevated absolute eosinophil counts at the time of insertion of the LVAD and the counts rapidly (<30 days) returned to normal after the operation. Because of the numerous medications that each patient was taking at the time of LVAD insertion, identification of a specific initiating medication as its cause was not possible. Of the 3 patients, 2 had idiopathic-dilated cardiomyopathy and 1 had ischemic cardiomyopathy and each had had heart failure for years. The eosinophilic myocarditis in these 3 patients appears to have been transient and superimposed on the earlier cardiomyopathy.