In an attempt to document the morphological progression from acute idiopathic myocarditis to end-stage dilated cardiomyopathy we studied 20 patients with a diagnosis of myocarditis who had had serial endomyocardial biopsies performed with intervals of 1 month to 2 years and whose ages varied from 6 months to 62 years. Fifteen of these patients were treated with immunosuppressive drugs for myocarditis. Ten out of 15 treated patients stabilized clinically. In the remaining 5 cases there was worsening congestive heart failure and 1 patient underwent cardiac transplantation. Of the 5 patients who did not receive immunosuppression, 2 stabilized spontaneously, and 3 developed heart failure, 2 of whom subsequently had cardiac transplants. Whether the patients received immunosuppression or not, in all cases, the inflammatory infiltrate was less but the myocardium developed significant hypertrophy with an increase in interstitial fibrosis and in 8 cases the morphological changes were those of dilated cardiomyopathy. From the morphological standpoint of this study we have shown some evidence that dilated cardiomyopathy can be the end result of acute myocarditis. It appears that not every case of acute myocarditis progresses to dilated cardiomyopathy and that steroid treatment does not necessarily prevent progression of myocarditis to dilated cardiomyopathy.