INTRODUCTION Trastuzumab, a recombinant humanized monoclonal antibody, is targeted against the external domain of the human epidermal growth factor receptor type 2 (HER2). It improves efficacy of HER2-positive breast cancer treatment. The authors present their experience with patients (pts) treated with trastuzumab in the aspects of cardiac complications. MATERIAL AND METHODS We observed prospectively 253 women with early positive HER2 breast cancer treated with trastuzumab. Assessment of cardiovascular status, ECG and echocardiography was performed initially and every 3 months until 6(th) month during follow-up. RESULTS Cardiac complications developed in 52 pts (20.55%) and included: asymptomatic left ventricle dysfunction (43), symptomatic heart failure (6), new asymptomatic LBBB (1); new negative T-waves in ECG (2). There was a progressive decline in left ventricular ejection fraction (LVEF) during treatment. It was more enhanced in pts with cardiac complications. Following trastuzumab termination/discontinuation LVEF increased but at month 18 still remained significantly lower than initially in both groups (61.07 ±4.84 vs. 59.97 ±5.23 - no cardiac complications; p < 0.05; 58.14 ±4.08% vs. 53.08 ±5.74% - cardiac complications; p < 0.05). During 6-month follow-up 33 out of 46 pts experienced an improvement in left ventricular status. In 13 pts in whom trastuzumab was discontinued, it was restarted; 6 of them successfully completed total therapy. Univariate analysis revealed no association between any cardiovascular risk factor and the development of cardiotoxicity. CONCLUSIONS One out of five treated patients discontinues trastuzumab in an adjuvant setting due to cardiac complications. LV dysfunction is the most frequent. Routine cardiac monitoring should be obligatory.