Trastuzumab improves clinical outcomes in patients with human epidermal growth factor receptor-2-positive breast cancer. However, cardiotoxicity is a potentially important concern, and the long-term cardiac effects of trastuzumab therapy remain unclear. Although reduction of cardiac function by trastuzumab is mostly reversible, some patients, especially those with cardiac risk factors, may rarely experience chronic heart failure or prolonged left ventricular ejection fraction reduction. There have been no detailed published analyses of patients with such unfavorable clinical courses. We report the rare case of a metastatic breast cancer in a woman without cardiac risk factors who experienced long-term irreversible cardiotoxicity after discontinuation of trastuzumab therapy.