Background: The gold standard endpoint in randomized clinical trials (RCTs) in MBC is OS, which has the disadvantage of requiring extended follow-up and being confounded by subsequent anti-cancer therapies. Although therapeutics have been approved based on PFS, its use as a primary endpoint is controversial. This study, the first IPD meta-analysis of targeted agents in MBC, aimed to collect data from RCTs of HER2-targeted agents in HER2+ MBC, assessing to what extent PFS correlates with, and may be used as, a surrogate for OS. Methods: A search was conducted in April 2011. Eligible RCTs accrued HER2+ MBC patients (pts) in 1992-2008. Collaboration was obtained from industrial partners (Roche, GSK) for industry-led studies. Investigator-assessed PFS was defined as the time from randomization to clinical or radiological progression, or death. A correlation approach was used: at the individual level, to estimate the association between PFS and OS using a bivariate survival model and at the trial level, to estimate the association between treatment effects on PFS and OS. Squared correlation values close to 1.0 would indicate strong surrogacy. Results: The search strategy resulted in 2137 eligible pts in 13 RCTs testing trastuzumab or lapatinib. We collected IPD data from 1963 pts in 9 RCTs. One phase II RCT did not have sufficient follow-up data so that 1839 pts in 8 RCTs were retained (5 evaluating trastuzumab, 3 lapatinib); 6 out of 8 RCTs were first-line. At the individual level, the Spearman rank correlation using Hougaard copula was equal to r=0.66 (95% CI 0.65 to 0.66) corresponding to an r² of 0.42. At the trial level, the squared correlation between treatment effects on PFS and OS was provided by R²=0.33 (95% CI -0.22 to 0.86) using Hougaard copula and R²=0.53 (95% CI 0.22 to 0.83) using log hazard ratios from Cox models. Conclusions: In RCTs of HER2-targeted agents in HER2+ MBC, PFS is moderately correlated with OS and treatment effects on PFS are modestly correlated with treatment effects on OS, similarly to first-line chemotherapy in MBC (Burzykowski et al JCO 2008). PFS does not completely substitute for OS.