Trastuzumab resistance hampers its well-known efficacy to control HER2-positive breast cancer. The involvement of PI3K/Akt pathway in this mechanism is still not definitively confirmed. We selected 155 patients treated with trastuzumab after development of metastasis or as adjuvant/neoadjuvant therapy. We performed immunohistochemistry for HER2, ER/PR, epidermal growth factor 1-receptor (EGFR), α -insulin-like growth factor 1-receptor (IGF1R), phosphatase and tensin homologue (PTEN), p110 α, pAkt, pBad, pmTOR, pMAPK, MUC1, Ki67, p53 and p27; mutational analysis of PIK3CA and PTEN, and PTEN promoter hypermethylation. We found 46% ER/PR-positive tumours, overexpression of EGFR (15%), α -IGF1R (25%), p110 α (19%), pAkt (28%), pBad (22%), pmTOR (23%), pMAPK (24%), MUC1 (80%), PTEN loss (20%), and PTEN promoter hypermethylation (20%). PIK3CA and PTEN mutations were detected in 17% and 26% tumours, respectively. Patients receiving adjuvant trastuzumab with α -IGF1R or pBad overexpressing tumours presented shorter progression-free survival (PFS) (all P ⩽0.043). Also, p110 α and mTOR overexpression, liver and brain relapses implied poor overall survival (OS) (all P ⩽0.041). In patients with metastatic disease, decreased PFS correlated with p110 α expression (P =0.024), whereas for OS were the presence of vascular invasion and EGFR expression (P ⩽0.019; Cox analysis). Our results support that trastuzumab resistance mechanisms are related with deregulation of PTEN/PI3K/Akt/mTOR pathway, and/or EGFR and IGF1R overexpression in a subset of HER2-positive breast carcinomas.