Purpose: Lapatinib, a dual tyrosine kinase inhibitor of epidermal growth factor receptor (EGFR/ErbB1) and human epidermal growth factor receptor 2 (HER-2/ErbB2), is effective against HER-2-positive locally advanced or metastatic breast cancer (MBC). This phase III trial evaluated the efficacy of lapatinib in HER-2-negative and HER-2-uncharacterized MBC. Patients and Methods: Women with MBC were randomly assigned to first-line therapy with paclitaxel 175 mg/m2 every 3 weeks plus lapatinib 1,500 mg/d or placebo. A preplanned retrospective evaluation of HER-2 status was performed using fluorescence in situ hybridization and immunohistochemistry. The primary end point was time to progression (TTP); secondary end points were objective response rate (ORR), clinical benefit rate (CBR), event-free survival (EFS), and overall survival (OS). Results: In the intent-to-treat population (n = 579), there were no significant differences in TTP, EFS, or OS between treatment arms, although differences in ORR and CBR were noted. In 86 HER-2-positive patients (15%), treatment with paclitaxel-lapatinib resulted in statistically significant improvements in TTP, EFS, ORR, and CBR compared with paclitaxel-placebo. No differences between treatment groups were observed for any end point in HER-2-negative patients. The most common adverse events were alopecia, rash, and diarrhea. The incidence of diarrhea and rash was significantly higher in the paclitaxel-lapatinib arm. The rate of cardiac events was low, and no difference was observed between treatment arms. Conclusion: Patients with HER-2-negative or HER-2-untested MBC did not benefit from the addition of lapatinib to paclitaxel. However, first-line therapy with paclitaxel-lapatinib significantly improved clinical outcomes in HER-2-positive patients. Prospective evaluation of the efficacy and safety of this combination is ongoing in early and metastatic HER-2-positive breast cancer patients. © 2008 by American Society of Clinical Oncology.