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e12009 Background: This study describes patterns of care and outcomes in metastatic breast cancer (MBC) patients treated with trastuzumab (T) within the US Oncology network. Methods: This retrospective study utilized data from US Oncology's iKnowMed EMR system. HER-2 (+) MBC patients who initiated a first-line regimen containing T between January 1, 2006 and July 31, 2007, were identified. Patients were divided into three treatment cohorts: A) those who discontinued T prior to disease progression; B) those who continued T following discontinuation of chemotherapy prior to progression; and C) those who received T monotherapy. Patients were followed through October 31, 2008, to measure treatment duration and observe progression (as defined as escalation to second-line therapy). The Kaplan Meier method/log-rank test were used to estimate and compare progression free survival (PFS) across cohorts. Results: We identified 139 patients receiving first-line therapy including T. The median age was 58 years and 84 patients were ER and/or PR (+). The top 5 chemotherapy regimens included T plus: paclitaxel (n = 14); docetaxel (n = 14); vinorelbine (n = 14); carboplatin/paclitaxel (n = 12); carboplatin/docetaxel (n = 7 pts). Twenty-one (15%) patients were in cohort A, 55 (40%) were in cohort B, and 63 (45%) were in cohort C. Overall, the median duration of first-line T use was 281 days (mean = 287; range = 1–862). Following the initiation of first-line therapy, a total of 56 (40%) patients progressed (median follow up = 15 months). Following progression, 22 (39%) patients received second-line regimens containing T. The overall median PFS was 17.4 months (mean = 18.6). Patients in cohort A (n = 15) had significantly shorter PFS versus patients in cohort B/C (median 3.9 and 19.6 months, respectively; p < 0.001). Conclusions: In this observational study within the outpatient community setting, persistent first-line T use for MBC was associated with delayed disease progression. Future research should evaluate the causal relationship of this association. In addition, a more comprehensive evaluation should be conducted of therapies prescribed concomitant to T for patients treated with T “monotherapy.” [Table: see text]