© 2014 All rights reserved: reproduction in whole or part not permitted.Introduction: Trastuzumab is the first anti-HER-2 humanized monoclonal antibody. The benefit of adjuvant trastuzumab has been shown in randomized phase III trials. Despite trastuzumab being recommended for 52 weeks in the adjuvant treatment of HER-2 positive breast cancer according to the current breast cancer guidelines, there is still no consensus on the optimal duration of adjuvant trastuzumab. The aim of our study is to investigate the efficacy and safety of adjuvant trastuzumab for 9 weeks and 52 weeks in axillary lymph node positive HER-2 positive breast cancer patients.Patients and methods: A total of 271 HER-2 and axillary node positive breast cancer patients who received trastuzumab in adjuvant treatment between the years 2005 and 2013 were retrospectively analyzed. Patients with axillary node positive HER-2 positive breast cancer who were non-metastatic were enrolled to the study. Patients were allocated to the 9 week trastuzumab group (n=155) or the 52 week trastuzumab group (n=116). Kaplan- Meier survival analysis was carried out for disease free survival (DFS) and overall survival (OS). Two-sided p values of <0.05 were considered statistically significant. The most important limitation of our manuscript is the retrospective design.Results: The median follow-up time for this analysis was 34 (4-95) months. Patients' clinical and pathological characteristics were well balanced between the two treatment arms. In the 9 week trastuzumab treatment group, the DFS rate was 96.7%, 84.8% and 74.9% in the first, third and fifth years respectively, whereas in the 52 week trastuzumab treatment group it was 94.3%, 80.0% and 80.0% (P=0.76). In the 9 week trastuzumab treatment group, the OS rate was 99.3%, 92.2% and 88.3% in the first, third and fifth years respectively, whereas in the 52 week trastuzumab treatment group it was 99.0%, 94.7% and 78.6% (P=0.99). In both groups, symptomatic heart failure was not reported but asymptomatic left ventricular ejection fraction (LVEF) decline was observed 3 (1.9%) and 18 (15.5%) patients in the 9 week and 52 week trastuzumab treatment groups, respectively (P<0.001).Conclusion: In our study, the efficacy of trastuzumab for 52 weeks and 9 weeks was similar in node-positive HER-2 positive breast cancer. Cardiotoxicity was significantly increased in the 52 week trastuzumab arm compared to the 9 week trastuzumab arm.