Background Anthracyclines are potent chemotherapeutics burdened by their cardiotoxicity. So far no marker to detect early cardiac damage exists. We tested the ability of magnetic resonance imaging (MRI) to show early changes in myocardial signal and cardiac function after anthracycline therapy. Methods Twenty-two patients with normal cardiac function were investigated by MRI before and 3 and 28 days after anthracycline chemotherapy. Contrast enhanced fast spin echo images were obtained to characterize myocardial enhancement. Left ventricular ejection fraction was measured by MRI in contiguous short-axis planes. Results All patients remained clinically stable. Ejection fraction decreased from 67.8% ± 1.4% to 58.9% ± 1.9% after 28 days (P < .05). The relative myocardial contrast enhancement increased from 3.8 ± 0.4 to 6.9 ± 1.1 (P < .01). An increase of the enhancement of >5 on day 3 compared with baseline predicted a significant loss of ejection fraction at 28 days (67.5% ± 2.8% to 51.4% ± 5.6%, mean difference 16.1% ± 6.6%; P < .05), whereas an increase of +5 was not associated with a significant loss of ejection fraction (67.6% ± 1.7% to 62.5% ± 1.4%, mean difference 4.1% ± 2.6%; P not significant). Conclusions MRI detects early changes in myocardial contrast and slightly deteriorating cardiac function in patients receiving anthracyclines. Larger patient cohorts and longer follow-up are needed to evaluate MRI as a predictor for anthracycline cardiotoxicity. (Am Heart J 2001;141:1007-13.)