Two cases in which right ventricular endomyocardial biopsy established a definitive pathologic diagnosis of acute myocarditis are presented. Both patients presented with severe left ventricular dysfunction, atrial flutter and an intraventricular conduction defect. Immunofluorescent and immunoperoxidase examination of the biopsy specimens suggested a humoral immune mechanism in one case and a cell-mediated immune mechanism in the other. Substantial improvement in cardiac function followed treatment with prednisolone and azathioprine in both cases. Significant regression of the myocardial inflammatory changes was documented by a further endomyocardial biopsy in each case following clinical improvement.