Trastuzumab added to chemotherapy is the cornerstone of adjuvant treatment of early HER2 positive breast cancer. Clinical trials and metaanalyses of adjuvant trastuzumab have shown significant reduction in risk of recurrence and death. Nevertheless, the real magnitude of the effect of any drug must be reevaluated in daily clinical conditions, due to the fact that daily clinical practice often differs from conditions in clinical trials. In order to measure the benefit of adding adjuvant trastuzumab in HER 2 positive early breast cancer treatment, we have performed retrospective analysis in a single institution on consecutive patients divided in two cohorts: one, treated in "pre-trastuzumab" and the other in "trastuzumab era". Between 2003 and 2012, 258 consecutive HER 2 positive patients with early breast cancer have been treated with adjuvant chemotherapy, 103 patients did not receive trastuzumab (patients treated from 2003 till 2007), and 155 received trastuzumab (patients treated from 2008 till 2012). Patients who received trastuzumab experienced significantly longer median disease-free survival (107 months vs 92 months, LR: 11,6, P<0,001); breast cancer-specific survival (130 months vs 117 months, LR: 10,7, P<0,001) and median overall survival (123 months vs 108 months LR: 11,6, P<0,001). The benefits of adding trastuzumab were independent of chemotherapy regimen and endocrine therapy. This retrospective analysis has shown a clear, statistically significant benefit of adjuvant trastuzumab in treatment of early HER2 positive breast cancer in daily clinical practice and confirmed the results of the registration clinical trials.