OBJECTIVES: To evaluate the economic impact of trastuzumab treatment in Metastatic Breast Cancer (MBC). Trastuzumab therapy is initiated in MBC patients over-expressing HER2. The product is licenced in monotherapy for patients pre-treated with anthracyclines and taxanes, or associated with paclitaxel for patients pre-treated by anthracyclines. METHODS: HERMES is a phase IV multicentric prospective study funded by the French ministry of health, evaluating the clinical, biochemical and pharmaco-economic aspect of trastuzumab treatment on MBC. HER2 status was determined by Immuno-histochemistry or FISH methods and H-ECD (HER2 Extra-Cellular Domain) status by ELISA technique. Only HER2 3+ or 2+ and FISH+ patients received treatment. Four protocols were administered: trastuzumab + paclitaxel weekly (TP1) or three weekly (TP3) and trastuzumab weekly (T1) or three weekly (T3). Responses were evaluated according to RECIST criteria then compared to H-ECD levels. Treatment costs were calculated by adding DGR costs (2004) and onerous drug reimbursed over DGRs. RESULTS: In a 3-years period, 120 patients were pre-included and 88 included. In intention to treat there were 62 TP1, 25 TP3, 1 T1. Time to Treatment Failure is 30 weeks [25-35]. 81 patients stopped treatment: 67% for progression, 16% for cardiac toxicities. Overall smurvival is 60 weeks [48-80]. Time to Progression is 34 weeks [27-43]. After 2 months, on 27 patients, relative risk of progression is of 2.2 for patients with H-ECD increase. On 22 patients with H-ECD diminution, 20 were responding to treatment. Overall patient management cost is of €4,178,000. Average pre-inclusion screening cost is of €829 per patient. Average treatment cost on 36 weeks reaches €46,345 per patient including 72% for drug acquisition, 23% for administration, 1% for laboratory assessments, 3% for cardiac assessment, 1% for tumour volume assessment. CONCLUSIONS: From an economic perspective, HER2 assays are cost effective: they are less expensive than cytotoxic and/or trastuzumab treatments.