Doxorubicin (Adriamycin) was administered by continuous infusion to reduce peak plasma levels and thus lessen cardiac toxicity. Cardiotoxicity was monitored by noninvasive methods, and endomyocardial biopsy specimens were studied by electronmicroscopy. Cardiotoxicity was compared in 21 patients receiving doxorubicin intravenously over 48 or 96 hours and in 30 control patients treated by standard intravenous injection. Both groups were studied prospectively and were well matched by risk factors for doxorubicin cardiotoxicity