1015 Background: Lapatinib (L) is an oral, small-molecule inhibitor of EGFR and HER2 with a mechanism of action distinct from that of trastuzumab (T). Preclinical data suggest synergy between L and T. We studied L alone and in combination with T in pts with HER2+ MBC who progressed on T. Methods: Eligible women had received prior anthracycline and taxane therapy, had MBC with measurable lesions or bone-only disease, and had progressed on prior T-containing therapy. Pts were stratified by hormone receptor status and visceral/nonvisceral disease, then randomized to receive either L (1,500 mg QD) or L (1,000 mg QD) plus T (2 mg/kg weekly after 4 mg/kg loading dose). If pts progressed on the L arm, they could cross over to the L+T arm. The primary endpoint was PFS (investigator assessment), and secondary endpoints were clinical benefit rate (CBR) at 24 wks, RR, and OS. Results: 296 pts were randomized. All pts had received prior T; the median number of prior chemotherapy regimens was 6. Combination therapy si...