Background: In patients with HER2-positive metastatic breast cancer (MBC) previously treated with trastuzumab and taxanes, there are now two standard strategies: trastuzumab beyond progression or switch to lapatinib. A randomized trial comparing trastuzumab plus capecitabine (HX) and capecitabine alone (X) after first-line trastuzumab-based chemotherapy (GBG-26) showed that HX was superior to X in terms of time to progression (TTP). Another randomized trial comparing lapatinib plus capecitabine (LX) and X in patients previously treated with anthracyclines, taxanes, and trastuzumab (EGF100151) showed that LX was superior to X in terms of TTP. To evaluate which strategy is better, we are conducting an open-label, randomized phase II trial comparing HX and LX. Methods: Primary endpoint is progression-free survival, and secondary endpoints are overall response rate, overall survival, proportion of patients progressing brain metastases as site of first progression, and safety. Major eligibility criteria include: (1) HER2-positive MBC, (2) previously treated with taxanes, (3) disease progression or distant relapse while receiving trastuzumab, (4) previously untreated with capecitabine, S-1, and anti-HER2 drugs other than trastuzumab, (5) previously treated with no more than two chemotherapy regimens for MBC, (6) no symptomatic brain metastases (asymptomatic brain metastases are allowed), and (7) baseline left ventricular ejection fraction ≥50%. Patients in the HX arm receive capecitabine 2,500 mg/m²/day on days 1 to 14 plus trastuzumab (8 mg/kg loading dose and 6mg/kg thereafter) on day 1 every 3 weeks. Patients in the LX arm receive capecitabine 2,000 mg/m²/day on days 1 to 14 plus lapatinib 1250 mg/day on days 1 to 21 every 3weeks. Large-scale biomarker analyses are also performed to explore predictive factors of trastuzumab or lapatinib efficacy. We are investigating biomarkers related to HER family and other receptors, PI3K/Akt pathways, ligands, FcγR, circulating tumor cells, and so on. This study has just begun and 7 of planned 170 patients have been enrolled.