Trastuzumab-related cardiac dysfunction may be manageable and completely reversible with suitable cardiac medication, allowing optimal breast cancer treatment to continue. We present the case of a 42-year-old woman who developed severe systolic left ventricular failure with impaired contractility of the right ventricle, pulmonary hypertension, and clots in the left ventricular cavity during adjuvant treatment for breast cancer. The patient was initially diagnosed with early breast cancer and underwent surgery on her left breast. She received 6 cycles of anthracycline chemotherapy followed by radiation therapy in the left breast area, then 5 cycles of trastuzumab. After the fifth cycle of trastuzumab, she experienced dyspnoea and leg edema. Fluid was detected in the pleural cavities but no lung metastases were identified. Echocardiography was performed, revealing a severely reduced left ventricular ejection fraction (10%) with impaired contractility of the right ventricle and pulmonary hypertension. Standard medication for heart failure resulted in complete recovery of normal systolic and diastolic function of the left and right ventricles. The combination of low molecular weight heparin and acetylsalicylic acid completely resolved the thrombotic complications. The patient regained her full range of social, occupational, and family activities. This case study is the first to demonstrate the manageability and reversibility of trastuzumab-related cardiac complications in a patient who had developed severe heart failure complicated with left ventricular thrombosis during sequential anthracycline and trastuzumab therapy for breast cancer. The findings contradict other opinions that trastuzumab-related acute heart failure is analogous to stunning or hibernation and recovers without specific cardiac treatment.