A line of nonobese diabetic (NOD) mice expressing the human diabetes-associated HLA-DQ8 transgene in the absence of mouse IA_β failed to show spontaneous insulitis or diabetes, but rather developed dilated cardiomyopathy, leading to early death from heart failure. Pathology in these animals results from an organ- and cell-specific autoimmune response against normal cardiomyoctes in the atrial and ventricular walls, as well as against very similar myocytes present in the outermost muscle layer surrounding the pulmonary veins. Progression of the autoimmune process could be followed by serial ECG measurements; irradiation of young animals significantly delayed disease progression, and this effect could be reversed by adoptive transfer of splenocytes taken from older animals with complete heart block. Disease progression could also be blocked by cyclosporin A treatment, but was accelerated by injection of complete Fruend's adjuvant. The constellation of findings of spontaneously arising destructive focal lymphocytic infiltrates within the myocardium, rising titers of circulating anticardiac autoantibodies, dilation of the cardiac chambers, and gradual progression to end-stage heart failure bears a striking resemblance to what is seen in humans with idiopathic dilated cardiomyopathy, a serious and often life-threatening medical condition. This transgenic strain provides a highly relevant animal model for human autoimmune myocarditis and postinflammatory dilated cardiomyopathy.