Rapid histological changes in endomyocardial biopsy specimens after myocarditis. The course and response to treatment in acute lymphocytic myocarditis are conventionally monitored by endomyocardial biopsy performed every 3-12 weeks. A patient with a short history (five days) of acute myopericarditis of unknown aetiology presented in cardiogenic shock with evidence of severe systolic dysfunction on the echocardiogram. The initial biopsy specimen showed histologically unequivocal myocarditis. Repeat endomyocardial biopsy after four days of treatment with steroids and azathioprine showed substantial histological improvement, a reduction in cellular infiltrate and myocardial necrosis, and interstitial fibrosis. Serial biopsies at 2 weeks and then 1, 2, 4, 5, 8, and 14 months after the initial biopsy showed progressive clearing of cellular infiltrate, increasing interstitial fibrosis, and compensatory myocyte hypertrophy by 4 months. At 14 months scattered lymphocytes persisted but myocyte abnormalities had resolved completely. The patient remained symptom free and systolic function was normal during this recovery period. Early endomyocardial biopsy (within one week of diagnosis) may yield useful histological information on the response to treatment in patients with myocarditis. It may not be necessary to wait the customary 3-4 weeks to repeat the biopsy. This case shows the chronology of histological changes and emphasises that a return to normal myocardial function may precede resolution of the histological abnormalities, which may persist in part or may resolve totally after the acute episode. 