The monoclonal antibody trastuzumab is at present the only modern targeted treatment which has been studied in early breast cancer. Data are available from four large and one smaller randomised trials where patients with HER2-positive operable breast cancer were randomised to trastuzumab or control in addition to standard adjuvant chemotherapy and endocrine treatment. Trastuzumab therapy resulted in relative risk reductions of 33-59% for recurrence and of 33-41% for death. Trastuzumab has become an integral part of adjuvant therapy in HER2-positive operable breast cancer.