Introduction: Trastuzumab-related cardiotoxicity has been a major concern in clinical practice, since observational studies have shown higher incidences than that reported in clinical trials. We aim to measure the incidence of trastuzumab-related cardiotoxicity in patients with early and metastatic breast cancer in the south of Brazil. Methods: Multicenter prospective observational study, which included 109 patients with early or metastatic HER-2+ breast cancer undergoing any trastuzumab-based regimen. Cardiac events were measured by transthoracic echocardiography assessments and by signs and symptoms of heart failure. Results: Trastuzumab-related cardiotoxicity was observed in 58 patients (53.2%). Emergency and hospitalization admissions were necessary in seven and three patients, respectively, due to symptoms of heart failure. One patient died in consequence of trastuzumab-related cardiotoxicity. In total, trastuzumab was discontinued in 31.2% of patients, of which almost a third could not return to treatment. In this study, no risk factors were significantly associated with the development of cardiotoxicity. Discussion: The incidence of TRC and trastuzumab's early discontinuation observed was significantly higher in comparison with other studies. These findings endorse the fact that trastuzumab-related cardiotoxicity is a relevant adverse reaction, and therefore, cardiac dysfunction's monitoring must be highlighted in order to allow a safe use of trastuzumab in this population.