Transient electrocardiographic changes and arrhythmias are known to be acute manifestations of cardiotoxicity secondary to cancer therapy with anthracyclines or cardiac irradiation. However, despite the known risk of late cardiac dysfunction in survivors of childhood cancer therapy, the risk of clinically important electrocardiographic abnormalities and arrhythmias after treatment is unknown. Standard 12-lead and 24-hour ambulatory electrocardiograms were recorded in 73 patients who received anthracyclines alone, 24 who received cardiac irradiation alone, and 27 who received both anthracyclines and cardiac irradiation. The mean age of the patients was 15 years. Mean cumulative anthracycline dose was 282 mg/m2 in patients who received anthracyclines alone and 244 mg/m2 in patients who received both anthracyclines and cardiac irradiation. Analysis of the 12-lead and 24-hour electrocardiograms demonstrated increased frequency of QTc prolongation, supraventricular premature complexes, supraventricular tachycardia, ventricular premature complexes, couplets and ventricular tachycardia (all p less than 0.001) when compared with an age-matched healthy population. Most patients had abnormalities limited to single supraventricular or ventricular premature complexes; however, potentially serious ventricular ectopy, including ventricular pairs and ventricular tachycardia, were noted in patients with cumulative doses greater than 200 mg/m2. Electrocardiographic abnormalities and arrhythmias are not limited to the acute phase of treatment with anthracyclines and cardiac irradiation. Survivors of childhood malignancy who received anthracyclines or cardiac irradiation, or both, probably should undergo ambulatory electrocardiographic monitoring as part of their follow-up to detect potentially life-threatening arrhythmias.