Repeat endomyocardial biopsy was performed in 28 patients with dilated cardiomyopathy of less than or equal to 12 months' duration and either symptomatic heart failure or life-threatening ventricular arrhythmias. Myocarditis was strongly suspected clinically in all cases, yet was unconfirmed on initial right ventricular biopsy. Seventeen patients underwent both right and left ventricular biopsy, seven patients had a repeat right ventricular biopsy and four patients underwent repeat left ventricular biopsy alone. The interval between initial and repeat biopsy averaged 31 +/- 6 days. Myocarditis was confirmed on repeat biopsy in 4 of 6 patients whose initial biopsy revealed "borderline" myocarditis (that is, interstitial inflammation but absence of myocyte necrosis) compared with none of the 22 patients whose initial biopsy showed either myocyte hypertrophy or interstitial fibrosis, or both (p = 0.0007). "Borderline" myocarditis on initial biopsy was the only clinical or histologic finding predictive of myocarditis on subsequent biopsy. Repeat endomyocardial biopsy can identify and potentially modify the treatment of an additional group of patients with dilated cardiomyopathy and nondiagnostic initial endomyocardial histologic features. Right ventricular sampling should be repeated in patients whose initial biopsy demonstrates "borderline" myocarditis.