Fifteen infants and children with dilated cardiomyopathy underwent transvascular endomyocardial biopsy. The light and electron microscopic findings were reviewed to evaluate the presence of lymphocytes as an indicator of active myocarditis. Both ventricles were biopsied in 13 patients, and the right ventricle only was biopsied in 2. None of the endomyocardial specimens obtained by biopsy revealed an inflammatory process. Interstitial fibrosis, myofiber hypertrophy, degeneration and necrosis were found. Ultrastructural abnormalities of the mitochondria, T tubules or Z bands were noted in approximately one-third of patients. Persistent, active myocarditis is an uncommon cause of dilated cardiomyopathy in children. Immunosuppressive therapy, which may be harmful, should be considered only after myocardial inflammation has been documented by endomyocardial biopsy.