The influence of increasing doses of doxorubicin on the heart was examined in 30 patients with solid tumors, M-mode echocardiography being used to evaluate left ventricular contractility. The function of the left ventricle remained normal in 26 subjects, whereas four patients had evidence of cardiotoxicity after cumulative doses of 220, 380, 420, and 450 mg/m2. Transient overt heart failure was noted in one subject only. Doxorubicin cardiotoxicity can be detected by M-mode echocardiography, a simple and non-invasive technique, prior to the appearance of overt congestive heart failure. Patients demonstrating left ventricular dysfunction are probably not candidates for receiving further therapy with anthracycline antibiotics. Limitation of M-mode echocardiography include a 28% incidence of inadequate studies in this group of patients, and a relative inaccuracy of the technique in evaluating patients with prior myocardial infarction.