This was a Phase II, randomized, multicenter, international, 2-arm, open-label clinical trial designed to explore the efficacy and safety of trastuzumab emtansine (T-DM1) relative to the combination of trastuzumab and docetaxel in patients with human epidermal growth factor receptor 2 (HER2)-positive, unresectable, locally advanced breast cancer and/or metastatic breast cancer who have not received prior chemotherapy for metastatic disease.