Human epidermal growth factor receptor-2 (HER2) is a member of the epidermal growth factor receptor family, which produces factors that are considered to be important mediators of cell growth. Overexpression of HER2, which occurs in approximately 25% to 30% of human breast cancers, has fostered considerable interest in innovative therapeutic modalities designed to target tumor cells demonstrating such overexpression. Trastuzumab (Herceptin; Genentech, San Francisco, CA), a humanized monoclonal antibody developed to target the HER2 receptor, is the most widely studied example of such a modality. In early clinical studies with trastuzumab, cardiomyopathy was observed with a clinical expression similar to that seen with the anthracyclines (ie, a potentially progressive decrease in cardiac systolic function). A number of possible explanations for this cardiotoxicity are explored in this report. The first is that trastuzumab has inherent toxicity. This consideration has some theoretical interest, since fetal myocardial cells exhibit HER2 receptors and the adult myocardium expresses HER3 receptors. A second possibility is that sequential stresses following doxorubicin administration contribute to cardiac dysfunction. A third explanation is that observational artifacts lead to an overestimation of trastuzumab cardiotoxicity. Approaches for additional study of the extent and severity of trastuzumab cardiotoxicity are briefly addressed.