The primary goal of therapy for metastatic breast cancer is to improve the outcome for patients. Ideally, this should be achieved with minimal short-term side effects and without long-term irreversible toxicity. Trastuzumab (Herceptin; F. Hoffmann-La Roche, Basel, Switzerland) is proven to be efficacious in women with metastatic breast cancer who have HER2-positive disease. Data from pivotal clinical trials and postmarketing surveillance in women with metastatic breast cancer confirm that trastuzumab is also well tolerated with a low incidence of conventional chemotherapeutic side effects. Severe adverse events are confined to serious infusion-related reactions and cardiac issues, which are infrequent and readily managed. Patients at risk of these severe events can be identified before starting trastuzumab therapy. Ideally, treatment should also be convenient for the patient. This can be achieved through less frequent dosing. A 3-weekly trastuzumab schedule, with higher individual loading and maintenance doses than the conventional weekly schedule, has been investigated. This has similar efficacy, tolerability, and pharmacokinetics (exposure) to the weekly regimen, providing a convenient schedule.