Abstract Introduction. We assessed the incidence and timing of first cardiac events, impact on trastuzumab prescription, and role of left ventricular ejection fraction (LVEF) monitoring in daily practice of trastuzumab-treated patients with human epidermal growth receptor 2 (HER2)-positive early breast cancer. Methods. We included all patients with stage I–III breast cancer diagnosed in the early years (2005–2007) after the introduction of adjuvant trastuzumab in five hospitals in Southeast Netherlands. We studied the incidence and timing of cardiotoxicity in patients treated with adjuvant trastuzumab, using similar cardiac endpoints as in the Herceptin Adjuvant (HERA) trial. Results. Of 2,684 included patients, 476 (17.7%) had a HER2-positive tumor. Of these, 269 (56.9%) were treated with adjuvant chemotherapy, and of these, 230 (85.5%) also received trastuzumab. Cardiotoxicity was observed in 29 of 230 patients (12.6%). Twenty of the 230 patients (8.7%) had symptomatic cardiotoxicity, defined as a drop in LVEF of at least 10 percentage points and to below 50%, accompanied by symptoms of congestive heart failure. Trastuzumab was definitely discontinued because of supposed cardiotoxicity in 36 patients (15.6%), of whom only 15 (6.5%) had a significant LVEF drop. Of the 36 patients who prematurely discontinued trastuzumab (including the 29 in whom cardiotoxicity was observed), 84.8% stopped in the first 6 months. No cardiac deaths were seen. Conclusion. In the first years after implementation of trastuzumab for treatment of early breast cancer, physicians frequently based their decision to discontinue treatment on patient symptoms apart from LVEF outcome. We suggest that focusing LVEF monitoring on the first 6 months might be more cost-effective without compromising patient safety. Nonetheless, further research is needed.