Clinical and instrumental (ECG, PCG, ultrasonic cardiography and polycardiography) examination of cardiac activity in 98 breast cancer patients treated at the Center after Cooper established the cardiotoxic effect of 5-fluorouracil, vincristine, methotrexate, cyclophosphamide and corticosteroids. 46% of them suffered pain in the region of the heart, tachycardia, extrasystole, atrial flutter and deranged conduction function. Congestive heart failure was observed in 7% only. In 24%, myocardial lesions could be detected by instrumental means only. They were identified on the basis of an increase in terminal diastole and systole volume and mass of the myocardium matched by a decrease in stroke volume, PCG amplitude, % delta S and VCF. Cardiac disorders did not persist and were hardly detectable during medication course intervals or one-two months after treatment.