Trastuzumab, a drug targeting human epidermal growth factor receptor 2, improves survival rate in women with metastatic breast cancer. Symptomatic heart failure, a serious adverse effect of trastuzumab, occurs in 1% to 4% of patients treated with the antibody, whereas left ventricular ejection fraction declines substantially in 10% of patients. The prevalence of cardiotoxic effects of trastuzumab appears to increase with exposure to anthracyclines. Serial assessment of left ventricular function with 2-dimensional echocardiography or radionuclide ventriculography is the most practical means of monitoring cardiotoxicity. Patients who develop cardiotoxicity while receiving trastuzumab therapy generally improve once use of the agent is discontinued.