The HER2/neu gene is amplified in approximately 25% of breast cancers, leading to HER2 protein overexpression and shortened overall survival and time to relapse. Trastuzumab is a humanised, monoclonal antibody against HER2, which improves survival for women with metastatic HER2-overexpressing breast cancer and reduces the risk of recurrence in women with early stage HER2-overexpressing breast cancer. Cardiac toxicity was an unexpected finding in the pivotal Phase III trial leading to the approval of trastuzumab, and prospective cardiac monitoring has, therefore, been incorporated into more recent clinical trials of trastuzumab. This article reviews the cardiac toxicity findings in key trastuzumab clinical trials and clinical characteristics of trastuzumab-associated cardiac toxicity.