Background: The relative efficacy of L vs T when combined with Tax chemotherapy in the first-line setting of metastatic breast cancer (BC) is unknown. Methods: MA.31 compares Tax-based therapy, paclitaxel 80mg/m2 wkly or docetaxel 75mg/m2 3 wkly for 24 wks in combination with L or T. The L dose was 1,250 mg po daily with Tax followed by 1,500 mg daily (LTax/L). After a loading dose, the T dose was 2 mg/kg wkly or 6 mg/kg 3 wkly + Tax followed by T 6 mg/kg 3 wkly (TTax/T). Stratification was by prior neo/adjuvant HER2 therapy, prior neo/adjuvant Tax, planned Tax (paclitaxel vs docetaxel), and liver metastases. The primary endpoint is ITT progression-free survival (PFS), defined as time from randomization to objective progressive disease based on RECIST criteria or death from any cause. The protocol-specified IA was performed after observing 333 PFS events; the trial was to stop if the 2-sided p-value from the stratified log-rank test was less than 0.03. The NCIC CTG’s independent DSMC reviewed IA results and recommended disclosure because the superiority boundary was crossed. A secondary analysis utilized central laboratory-confirmed HER2 + status. Results: Between July 17 2008 and Dec 1 2011, 652 pts were accrued. Data from 636 pts (525 HER2 centrally confirmed) were included in the IA with clinical cutoff date of Nov 7 2011 and database lock of Apr 13 2012. Median follow-up was 13.6 mos, 12.9 mos for LTax/L pts and 14.0 mos for TTax/T patients. In the ITT analysis, PFS was inferior with LTax/L compared to TTax/T hazard ratio (HR) =1.33; 95% CI 1.06-1.67; p=0.01. LTax/L had median PFS 8.8 mos (95% CI 8.3-10.6) compared to TTax/T 11.4 mos (95% CI 10.8-13.7). PFS in the centrally confirmed HER2+ had HR 1.48 (95%CI 1.15-1.92; p=0.003) (LTax/L to TTax/T). No difference in overall survival was detected (LTax/L to TTax/T) HR= 1.1 (95% CI 0.75-1.61; p=0.62). More grade 3-4 diarrhea and rash was observed with LTax/L (p<0.001). Conclusions: LTax/L therapy is associated with a shorter PFS compared to TTax/T as first line therapy for HER2+ metastatic BC. ClinicalTrials.gov: NCT00667251. CCSRI grant: 021039. Supported by GlaxoSmithKline.