Anthracycline derivatives may produce early or late cardiotoxic reactions in man. Early effects include: (a) pericarditis-myocarditis which can affect patients with no previous history of cardiac disease and which carries a high mortality rate ( approximately 20%); (b) left ventricular dysfunction which may lead to clinically significant heart failure in patients with limited cardiac reserve; and (c) arrhythmias, the most common of which is sinus tachycardia. Symptomatic supraventriclar tachycardia, heart block, and ventricular arrhythmias can occur, however, and may reflect primary effects on cardiac muscle or the conduction system. Late effects of anthracyclines are directly related to the degree of associated myocyte damage and include subclinical left ventricular dysfunction and overt heart failure. The implications for prognosis and further treatment are discussed for each of these entities and a common pathogenetic mechanism is proposed.