To assess chemotherapeutically induced myocardial damage, myosin-specific antibody scans and ejection fraction measurements were performed in 32 patients with breast cancer and in 9 patients with other tumours. All patients had received chemotherapy including anthracyclines. The ejection fraction decreased by less than or equal to 10% in 14 of 41 (34%) patients after chemotherapy. Antimyosin uptake in the myocardium was observed in 38 of 41 (92%) patients after chemotherapy. Antimyosin uptake was quantified by means of a heart-to-lung ratio, revealing a correlation between the degree of antimyosin uptake in the myocardium and the cumulative dose of anthracycline. Patients with a decreased ejection fraction showed more intense antimyosin uptake, indicating more severe myocardial damage. A higher degree of antimyosin uptake was found in 17 breast cancer patients treated with doxorubicin compared with 15 patients treated with mitoxantrone. We conclude that antimyosin studies provide a sensitive, non-invasive method to monitor myocardial damage in patients treated with anthracyclines. Antimyosin uptake in the myocardium precedes ejection fraction deterioration. This technique may be helpful in the early identification of patients at risk of congestive heart failure during chemotherapy including anthracyclines.