Introduction: Trastuzumab is one of the most effective treatments in HER - 2 positive breast cancer patients. One of the important side effects of trastuzumab is cardiotoxicity, which usually occurs as a reduction in the left ventricular ejection fraction (LVEF). Discontinuing the treatment due to cardiotoxicity may lead to cancer progression and worsen prognosis. The aim of the cur rent study was to evaluate the cardiac function in HER - 2 positive breast cancer patients who were candidates for trastuzumab therapy but had already left ventricular dysfunction prior to initiation of the treatment. Methods: HER - 2 positive breast cancer patients with asymptomatic LV dysfunction with LVEF 40 - 53% who were candidates for receiving trastuzumab (after initial chemotherapy) included in this study. Patients were visited in the Cardio - Oncology clinic before initiating the treatment and then before every two cycles of trastuzumab. They also received the standard treatment for heart failure including a beta - blocker (carvedilol) and ACE - I (Lisinopril), up to the maximum tolerated dose, if there were no contraindications. Patients were followed up 6 months after the end of treatment. Myocardial infarction (MI), cardiac arrhythmia, heart failure(HF) symptoms and any death that occurred due to Cardiovascular diseases were recorded as cardiac events. If the LVEF decreased below 40%, the treatment was temporarily interrupted for one or two cycles, and spironolactone was added to the patient's treatment. but If the LVEF improved(=40%), trastuzumab was rechallenged. Results: Thirty - six patients were included in the study. LVEF reduction of more than 10% occurred in 16.7% of the patients, and a GLS reduction of more than 15% was detected in 11.1% of the patients. There was a significant association between a =10% reduction in LVEF and baseline systolic blood pressure (P - value: 0.04). LVEF reduction below 40% was observed in 3 patients, that trastuzumab was interrupted in them. All of these three patients had obesity and uncontrolled HTN, and one of them had symptoms of heart failure (NYHA class II), for whom the trastuzumab treatment was discontinued. In the two other patients, after the temporary interruption of trastuzumab, LVEF improved to above 40%, and the treatment was restarted with close cardiac monitoring; therefore, they could complete the entire one - year treatment period. Conclusions: Continuing the treatment with trastuzumab seems to be safe in patients with reduced LVEF (LVEF = 40 - 53%), but they should be strictly monitored and controlled for cardiovascular risk factors, especially HTN. [ABSTRACT FROM AUTHOR]