Background: Approximately one-half of HER2-positive breast cancer patients will respond tofirst-line trastuzumab-containing therapy. However, in those patients with an initial trastuzumab response, most will progress within a year with acquired resistance. Since trastuzumab treatment is also now used in the HER2-positive adjuvant breast cancer setting, trastuzumab resistance will continue to be a recurring clinical problem, and better predictive and prognostic biomarkers are urgently needed. Methods: Seven serum biomarkers (carbonic anhydrase 9 (CA9), endoglin, HER2, IGF-1R, tissue inhibitor of metalloproteinase-1 (TIMP-1), urokinase-type plasminogen activator (uPA), and VEGF-A (isoform 165) were measured using ELISA assays in 81 metastatic breast cancer patients before starting first-line trastuzumab-containing therapy. The endoglin and IGF-IR ELISAs were from R&D Systems; others were from WILEX/Oncogene Science, Cambridge, MA. PFS and OS were analyzed using the Kaplan-Meier method and Cox modeling with continuous pretreatment serum biomarker variables. Results: For univariate PFS analysis, higher pretreatment serum biomarkers (except IGF-1R and VEGF-A) predicted reduced PFS (p<0.05) to first-line trastuzumab-containing therapy. In multivariate PFS analysis, only serum CA9 (p= 0.038) remained a significant independent covariate. In univariate OS analysis, higher pretreatment serum biomarkers (except IGF-1R and VEGF-A) were prognostic for reduced OS (p<0.05). In multivariate analysis for OS, TIMP-1 (p=0.001) and CA9 (p=0.04) remained significant independent prognostic factors, as well as line of chemotherapy (3 vs. 2 or 1 line)(p=0.005), and hormone receptor status (ER and/or PR positive vs. negative)(p=0.013). Conclusions: Higher pretreatment serum CA9 (a marker of hypoxia) predicted reduced PFS, and higher serum CA9 and TIMP-1 predicted reduced OS in metastatic breast cancer patients treated with first-line trastuzumab-containing therapy. These serum biomarkers deserve further study in larger trials of HER2-targeted breast cancer treatment. Supported by a grant from Komen for the Cure.