The diagnosis of acute myocarditis has been a conundrum. One criterion is the clinical picture of the sudden onset of cardiac failure and arrhythmias, often associated with a febrile illness; the other criterion is a myocardial-biopsy specimen that shows a specified degree of myocyte damage and infiltration by T lymphocytes. Regrettably, the concordance between the clinical and histologic diagnoses is poor, possibly because the clinical diagnosis is wrong or the histologic criteria used by pathologists are inappropriate. Either way, the clinician is left in a quandary about how to treat patients with the recent onset of heart failure of uncertain . . .