© 2015 Informa UK Ltd.Purpose: Trastuzumab induced cardiotoxicity (TIC) was defined as the most serious side effect. Long term cardiac effects of trastuzumab are still not known, thus we aimed to compare the long term cardiac effects of adjuvant trastuzumab therapies of HER2-positive breast cancer according to the treatment duration. Methods: Patients who completed adjuvant trastuzumab treatment at least 6 months before for the adjuvant setting in HER2-positive breast cancer were included in the study. A total of 164 patients were included in this study: 108 and 56 patients were treated with 9 weeks and 52 weeks of trastuzumab, respectively. The main limitation of our study is that due to the cross-sectional evaluation of cardiac biomarkers we cannot predict the status of baseline cardiac biomarkers of this population. Results: The median follow-up of the study was 32 (10-95) months. The accompanying chronic diseases were similar in both groups. Baseline left ventricular ejection fraction (LVEF) was 65.5±3.4% vs 67.1±4.5% in the 9 weeks and 52 weeks trastuzumab treatment groups, respectively (p=0.13). Symptomatic heart failure was not observed during trastuzumab treatment in either group. Trastuzumab induced cardiotoxicity (TIC) was observed in 2 (1.9%) and 17 (30.3%) patients in the 9 and 52 weeks trastuzumab treatment groups, respectively (p<0.001). After a median 24 months of follow-up from the last dose of trastuzumab, mean LVEF values were similar between the two treatment arms (p=0.29). In the subgroup analyses, mean LVEF values were significantly lower in patients who developed TIC compared to those who did not develop TIC (61.9±3.6% vs 64.4±2.6%, p=0.04). Average mean LVEF loss from baseline was significantly higher in patients who developed TIC compared to those who did not develop TIC (10.0±6.0% vs 1.5±6.2%, p<0.001). Cardiac biomarkers were similar in both treatment groups. In the subgroup analyses serum High-sensitivity C-reactive protein (hs-CRP) and prohormone brain natriuretic peptide (pro-BNP) levels were significantly higher in patients who developed TIC compared to those who did not develop TIC. Conclusions: TIC was observed to be significantly higher in the 52 weeks trastuzumab group. At the end of 32 months of follow-up mean LVEF values and cardiac biomarkers were similar between the two treatment groups. In the subgroup analyses, significant LVEF loss and higher cardiac biomarkers which show cardiac damage in patients who developed TIC can be permanent in some of the patients and long term cardiac damage may be underestimated.