The purpose of this study was to characterize in detail the morphologic changes in one form of drug related myocarditis: hypersensitivity myocarditis. We reviewed the clinical histories and microscopic findings in 24 patients with validated drug hypersensitivity and myocarditis. The myocarditis in these patients was characterized by a patchy interstitial inflammatory infiltrate rich in eosinophils, focal myocytolysis, prominent perivascular infiltrates, an absence of myocardial fibrosis, and the uniform features of the myocarditis in each patient. Twenty of the 24 patients died suddenly and unexpectedly. In retrospect, the presence of signs and symptoms of hypersensitivity (skin rash, fever, eosinophilia, and malaise) and nonspecific cardiac findings (electrocardiographic changes, tachycardia, or serum enzyme elevations) were suggestive of hypersensitivity myocarditis in these patients. Since early diagnosis is vital in hypersensitivity myocarditis, the presence of nonspecific cardiac findings in a patient receiving any drug associated with hypersensitivity should alert the clinician to the possibility of drug related myocarditis.