Myocarditis was diagnosed by endomyocardial biopsy in 34 patients with otherwise unexplained heart failure. On the basis of both clinical and histologic findings these patients were divided into three groups. Seven patients had acute myocarditis (mean age, 20 years; mean ejection fraction, 22 per cent) characterized by an interstitial inflammatory infiltrate and extensive, acute cell damage. Five of these patients died after a mean duration of illness of eight weeks. Eighteen patients had rapidly progressive myocarditis (mean age, 35 years; mean ejection fraction, 19 per cent) characterized by patchy acute and healing cell damage and fibrosis; 17 of them died after a mean duration of illness of 23 months. Nine patients had chronic myocarditis (mean age, 31 years; mean ejection fraction, 31 per cent) characterized by focal inflammation and cell damage. All nine were alive after a mean follow-up period of 39 months. In four of these nine, clinical and hemodynamic improvement occurred after one month of immunosuppressive therapy. Our study suggests that a clinically useful classification of myocarditis can be accomplished by endomyocardial biopsy.