Breast cancer is the most common cancer in women worldwide, its incidence is increasing but mortality has decreased in a huge way due to the growth of various cancer treatments. Patients with a contraindication for trastuzumab, an incomplete cardiographic echo record, and patients with already heart disease were excluded from the history. The means of cardiac monitoring was cardiac ultrasound or isotopic ventriculography, the toxicity criteria were those of the New York Heart Association (NYHA), a detailed operating sheet was made for each of our patients. The main objective of our study was to assess the incidence of cardiotoxicity in patients treated for breast cancer HER 2 3+ by trastuzumab.06% of our patients had had trastuzumab cardiotoxicity (i.e. 3 cases); two patients had an asymptomatic decrease in reversible LVEF and one patient had clinically significant heart failure requiring appropriate treatment for ICS and permanent discontinuation of trastuzumab. The protocols of targeted therapy with trastuzumab are an essential progress in oncology and cardiac toxicity is the major limiting factor for its use. Our work focuses on a retrospective study, conducted at the Department of Oncology and the Hospital Ahmida Ben adjila discussing cardiotoxicity of tarstuzumab in 50 patients with breast cancer HER2 3+ collected from january 2014 to december 2019.we have included in this study all patient with breast cancer overexpressing HER2 3+ treated by trastuzumab with an initial heart function and preserved LVEF ≥5 5%. We excluded patients contre to trastuzumab, incomplete echocardiographic folder, patients who received neoadjuvant trastuzumab and patients with metastatic breast cancer. Cardiotoxicity with trastuzumab is not common enough in our series but it does exist, which requires several careful preventive measures including monitoring by echocardiography.