Several molecular mechanisms are believed to take role in the development of trastuzumab resistance in breast cancer patients with overexpressing HER2. However, the sequence and the activity of these mechanisms are still unclear. In this study, Rac1 and neuregulin 1 (NRG1), two of ErbB pathway related proteins, were analyzed in HER2-positive breast cancer patients to investigate their roles in trastuzumab resistance. Trastuzumab resistance in metastatic breast cancer treatment was defined as a progression within six months of the treatment and in the adjuvant manner as an occurrence of local/distant recurrence before completion of treatment. Expression of Rac1 and NRG1 by immunohistochemistry (IHC) was studied in all 22 (n=12 adjuvant, n=10 metastatic) trastuzumab-resistant and 27 control tissue samples. The staining intensity of Rac1 was statistically significant in adjuvant treatment resistant group when compared with the controls (p=0.02). In addition, when all resistant groups were compared with the control groups, Rac1 staining intensity was denser (p=0.051). NRG1 staining intensity was in tendency to be denser as compared to control group, however it did not reach to a statistically significant level (p=0.09). In HER2-positive breast cancer, presence of Rac1 protein is significantly associated with early response failure to adjuvant trastuzumab therapy. However, further studies with larger groups are warranted to show the value of these molecules in predicting the response to trastuzumab-based therapies.