 The aim of this study is to analyze cases of human epidermal growth factor receptor HER 2-positive metastatic breast cancer MBC of the last 10 years at the University Hospital of Zurich to assess the efficacy of the treatment with trastuzumab in HER2-positive MBC and to find out associations between different variables and the outcome. The aim is to find out probable prognostic factors and patterns of disease progression. Prognostic factors could optimize treatment approaches and result in a delay of disease progression. 