OBJECTIVE This observational study aimed at analyzing adherence to prescribing guidelines of anti-HER2 monoclonal antibody trastuzumab treatment for metastatic breast cancer. Efficacy and costs were also evaluated. METHODS The adherence to the trastuzumab treatment plan was analyzed according to both the French postlicensing guidelines published in 2001 and clinical guidelines from the regional cancer network in a cohort of 131 consecutive patients. RESULTS The level of appropriateness to the molecular target was very high (92% of the patients showed a positive HER2 status, defined as HER2 3+ confirmed by immunohistochemistry or 2+ confirmed by fluorescent in situ hybridization). The treatment plan was made according to the French postlicensing guidelines in 41 patients (31.3%) and to the regional clinical guidelines for 109 patients (83.2%). The main reason for the difference was the type of molecules authorized for combination to trastuzumab. The median overall survival of the studied population was 18.6 months and the median progression-free survival rate was 7.7 months. Up to death or end of the study, the overall cost for the treatment of breast cancer with trastuzumab per patient and per year was 47,832 euro. CONCLUSION This quite low adherence of clinicians to the French postlicensing guidelines is in contrast with the high level of adherence to the regional clinical guidelines. The reason is that the latter are less rigid about previously received treatments and enlarge the potential associated cytotoxics to vinorelbine. This supports the French National Cancer Institute decision to get expert clinicians involved together with the French agency for sanitary security of health products and the high health authority in a common elaboration of guidelines.