Purpose: Trastuzumab is the most widely prescribed anti-HER2 humanized monoclonal antibody. Cardiac toxicity is the only limiting toxicity of trastuzumab and it is of particular concern in patients with complete response, since the drug needs to be stopped, with a risk of disease relapse. To date, no pharmacological data on trastuzumab cardiotoxicity in patients have been made available. Here, we provide proof of concept, demonstrating that it was possible to prevent trastuzumab-induced cardiotoxicity by modifying the drug administration schedule. Methods: In this paper, we report the case of a patient with metastatic breast cancer responding to trastuzumab, who developed severe cardiac toxicity twice using a 3-weekly regimen. Considering preclinical pharmacological data on trastuzumab cardiotoxicity, we hypothesized that a weekly schedule of trastuzumab with lower peaks of serum concentration could be safe while remaining efficient. With the patient's consent, we started a weekly combination of carboplatin (AUC2) and trastuzumab (2 mg/kg) with close monitoring of trastuzumab concentrations. Results: We successfully controlled the disease for an additional 6 months with relevant trough concentrations of trastuzumab of around 50 mg/L. Another important aspect is that, with this weekly schedule, we observed no cardiac toxicity, and the left ventricular ejection fraction remained stabilized, at over 50%. Conclusions: Trastuzumab is the most widely prescribed anti-HER2 monoclonal antibody for the treatment of HER2 metastatic breast cancer, and it is the only drug that has been approved for the treatment of localized HER2 breast cancer, 1-year treatment being required after surgery. In case of cardiac toxicity, particularly in women over 60 years of age, a weekly regimen with lower peaks of concentration could be an alternative to the standard 3-weekly regimen.