One important approach to change the natural history of advanced breast cancer is that of designing new combination chemotherapies in which the best drugs already available are used together with new anticancer agents devoid of clinical cross-resistance. The possibility of exploiting the combined use of cetuximab and trastuzumab represents an option of potential great impact on the probability of response and long-term therapeutic results for patients with advanced breast cancer and HER2 overexpression.Therefore, patients with tumors that demonstrate EGFR expression and clear-cut erbB-2 overexpression (3+) or limited erbB-2 overexpression (+ or 2+) will be included in the study. Patients will be treated with trastuzumab and cetuximab to study the pharmacokinetics and potential drug/drug interaction of both antibodies as well as the safety and tolerability of this novel combination treatment.