Herceptin (Trastuzumab) is a humanized recombinant monoclonal antibody that binds the extracellular domain of the human epidermal growth factor receptor 2 (HER2) and is used in the treatment of patients with HER2 overexpressing metastatic breast cancer. Treatment with Herceptin is generally well tolerated. At times, however, it exerts cardiac toxicity especially when used in combination with anthracyclines. We evaluated cardiac function before and after Herceptin treatment in nine patients with metastatic breast cancer by echocardiography, measuring ejection fraction (EF) and deceleration time (DcT). EF was significantly reduced after treatment(P<0.05). Although the present study failed to show significant changes in DcT, definite diastolic disturbance of the left ventricle did occur in a couple of patients. We conclude that cardiac dysfunction may be a common side effect of Herceptin even in early stages of treatment and that echocardiography will be a useful means of monitoring cardiac function in these patients.