This randomized, two-arm study evaluated the efficacy and safety of a combination of trastuzumab and capecitabine with or without pertuzumab in patients with human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer. The study population consisted of female patients, whose disease had progressed during or following previous trastuzumab therapy for metastatic disease. All patients in Arm A and Arm B received trastuzumab (8 mg/kg iv as loading dose and then 6 mg/kg iv every 3 weeks thereafter) and capecitabine oral twice daily for 14 days every 3 weeks (1250 mg/m2 twice daily in Arm A and 1000 mg/m2 twice daily in Arm B). In addition, patients in Arm B received pertuzumab (840 mg iv as loading dose and then 420 mg iv thereafter) every 3 weeks. Study treatment continued until disease progression or unacceptable toxicity.