Resistance to trastuzumab in patients with HER2-overexpressing breast cancer is associated with higher risk of progression or cancer death, and might be related to activation of PI3K/AKT/mTOR and Ras/Raf/MAPK signaling cascades and a decreased level of their inhibitor (PTEN). HER2-overexpressing breast cancer patients (n=75) treated with radical local therapy and trastuzumab in adjuvant setting were included into the study. Deoxyribonucleic acid isolated from paraffin sections was used to assess mutational status of the PIK3CA gene (p.H1047R and p.E545K mutations) by the quantitative polymerase chain reaction technique. Expression of selected proteins (ER, PgR, AR, Ki-67, EGFR) was assessed using immunohistochemistry. In the studied group we found significantly higher Ki-67LI in EGFR-positive carcinomas (p=0.048). Moreover, EGFR immunonegativity was observed more frequently in low-grade (G1/G2) carcinomas as well as in estrogen/progesterone and androgen receptor immunopositive tumors (p=0.042, p=0.016, p=0.044, respectively). Favorable metastasis-free survival was observed in patients with pN0 and pN1 (vs. pN2+3) stage (p=0.040) and with tumors characterized by low Ki-67LI (≤50% vs. >50%) (p=0.014). Patients with tumor androgen receptor immunonegativity (weak or lack of expression) or strong PTEN expression survived 3 years without metastases (p=0.007). The results of our study suggest that androgen receptor and PTEN status might be considered as indicators of trastuzumab sensitivity.