Evaluation of the ultrastructure of material obtained by endomyocardial biopsy has been proposed as a means to evaluate patients for impending anthracycline cardiotoxicity. Eighteen biopsies obtained from 13 patients (age, 3-18 years) are reported. Twelve biopsy procedures were done to evaluate the cardiac status on reaching a cumulative dose of 400 mg/m2 and three patients had six subsequent biopsies after having received significantly more drug or receiving radiation therapy to the lungs or mediastinum. Scores were assigned to the tissue obtained and used to guide the decision to continue or stop anthracycline therapy. Three patients with abnormal cardiac studies at low cumulative doses, five of whom had received greater than 400 mg/m2 and three of whom had received considerably higher doses and thoracic irradiation were given more drug without incident. Two specimens were interpreted to indicate avoidance of further anthracycline and two patients were cautiously given more despite evidence of mild myocardial damage. These results indicate that endomyocardial biopsies can be performed on a pediatric population with a reasonable complication rate. Further studies should be undertaken to evaluate its usefulness as a means to predict and avoid anthracycline cardiomyopathy.