We aimed to investigate the cardiac changes in patients with human epidermal growth factor receptor 2 (HER2)-positive breast cancer treated with trastuzumab in an adjuvant setting. Two hundred and fifty-three women with HER2-positive breast cancer were included. The assessment of cardiovascular system and echocardiography were performed and compared at baseline, at the termination of trastuzumab therapy and 6 months latter. Left heart remodeling was defined arbitrary as the change in at least one of the analyzed echocardiographic parameters of ≥standard deviation (SD) (in model I) or ≥2×SD (in model II) after 6-month follow-up. After 6-month follow-up 39 (31.7%), 27 (22%), 14 (11.4%), 10 (8.1%), 5 (4.1%) and 1 (0.8%), women had at least one parameter with a change exceeding mean difference ≥SD, respectively; and 30 (24.4%), 9 (7.5%), 3 (2.4%), 2 (1.6%) 1 (0.8%) exceeding mean difference ≥2SD. In stepwise multivariate regression analysis sedentary life style (OR16.7, p=0.003), positive cardiovascular family history (OR 6,9; p=0.013) and left ventricular ejection fraction change after 3 months (OR 1.2; p=0.007) were independent predictors of left heart remodeling in model I, whereas hypertension (OR 5.6; p=0.06) and positive cardiovascular family history (OR 3.9; p=0.032) were independent predictors of heart remodeling in model II. In conclusion, trastuzumab induces LV and left atrial cavity dilatation together with LV systolic function impairment.