Trastuzumab is a recombinant humanized monoclonal antibody directed against the HER-2. HER-2 is overexpressed in 20-25% of invasive breast cancer and known as poor prognostic factor and its status of a tumor is related to response to trastuzumab. Combination therapy with trastuzumab and chemotherapy increases response rate, time to progression, and survival thus trastuzumab is perceived to be the first line treatment for HER-2 overexpressed metastatic breast cancer. The optimal duration and period of trastuzumab treatment are still unknown and result of large scale randomized trial will be expected.