Objective: HER2 positivity defines a clinically challenging subgroup of patients with breast cancer with variable prognosis and response to therapy. The main aim of this study is to identify immunohistochemical markers to predict trastuzumab resistence. Method: Tumours from 57 patients with Invasive Ductal Carcinoma, who were previously treated with trastuzumab, were included in a tissue microarray and stained for ER, Ki67, p53, cyclin D1, p16 and for HER2 and SISH (silver in situ hybridization). Results: After 5 years of follow up, 80 % of patients with p53 negative tumours were disease free (p 0.029) and 90 % were alive (p 0,014) and the patients cyclin D1 positive, 90 % were alive (p 0.031) and 84 % were disease free (p 0.005). Among HER2 3+, 10 % did not amplify (polysomy 17). Moreover, 12 % of treated patients were HER2 0−2+ and did not amplify by SISH. Conclusion: The best immunohistochemical markers to predict a good response to trastuzumab treatment were P53 negative and/or cyclin D1 positive. All HER2 positive 3+ tumours should be confirmed by hybridization, since polysomy 17 is found in 10 %, and these tumours should not be treated with trastuzumab. Variations in the immunostaining techniques could induce trastuzumab treatment to patients with HER2 non amplified tumours.