Numerous clinical studies have demonstrated the therapeutic benefit of trastuzumab in women with breast cancer. However, a small but not insignificant proportion of patients have experienced trastuzumab-associated cardiotoxicity during these trials. This phenomenon is generally characterized by an asymptomatic reduction in left ventricular ejection fraction (LVEF) or, less often, congestive heart failure (CHF). Concomitant anthracycline therapy significantly increases the risk for cardiotoxicity during trastuzumab treatment, and such regimens are therefore not recommended. The cardiac dysfunction associated with trastuzumab is most often reversible upon discontinuation of treatment and initiation of standard medical therapy for CHF. Prior to treatment initiation, a risk-benefit analysis should be performed for each individual patient, including a thorough assessment of potential risk factors and cardiac function. Cardiac monitoring should be continued throughout trastuzumab therapy and the follow-up period, because early recognition of trastuzumab-associated cardiac dysfunction can allow effective medical intervention. Following the occurrence of asymptomatic LVEF reduction or CHF and appropriate medical intervention, reintroduction of trastuzumab may be considered in patients following resolution of normal cardiac function, or in those for whom the benefit of antitumor therapy outweighs the risk for CHF.