Abstract Background: Less than 5% of adult cancer patients participate in trials, delaying the arrival of new therapies to the clinic. This is concerning for metastatic breast cancer patients, for whom there is no curative treatment. To facilitate metastatic patients' access to trials, BreastCancerTrials.org (BCT) partnered with five breast cancer advocacy groups to design and develop Metastatic Trial Search (MTS), a trial matching service based on BCT technology and embedded on their websites. Approach: BCT's partners include Breastcancer.org, Living Beyond Breast Cancer, Metastatic Breast Cancer Network, Triple Step Toward the Cure, and Young Survival Coalition. MTS was seamlessly integrated on each partner's website. To find trials, users submit data about their gender, age, location, menopausal status, cancer subtype, and sites with evidence of disease and in return receive a list of matching trials in BCT's patient-friendly format. MTS launched October 1, 2015. Results: Our evaluation included analysis of web traffic, an online user survey, two user focus groups, and partner interviews. MTS traffic between Oct. 2015 and May 2016 resulted in over 10,000 page views; peaks of activity correlated with social media posts. The user survey, conducted between Oct. 2015 and Jan. 2016, had 102 participants; 88% learned about MTS from our advocacy partners, mostly through email (52%) or social media (21%). Most participants (60%) cited learning about new breast cancer research as an important benefit of MTS. Sixty-three percent of users were looking for treatment trials, 16% contacted a research site and 12% spoke to their doctors about trials that they found on MTS. Among all respondents, 7% enrolled in a trial and 29% were still considering enrollment. Sixty percent of MTS users were satisfied with their experience, 33% were neutral, and 6% were dissatisfied; 65% would recommend MTS to a friend. To improve MTS, users ranked adding filters to narrow search results as the most pressing need. Providing a trial alert service was ranked second. Over 80% agreed with the statement: “MTS met an unmet need.” The focus groups were made up of 14 survey respondents who volunteered to participate. Discussants spoke highly about MTS' ease-of-use and its advocacy group endorsement. They also appreciated how MTS helped them learn about different ways to participate in research and provided information to share with their oncologists. According to many participants, one of the most important benefits of MTS was that it helped them “be prepared” if they needed to change treatment. Our advocacy partners were very satisfied with their involvement and all stated that “collaboration and sharing” were the most important outcomes. In addition, they viewed the consistency of seeing MTS on each of their sites as a benefit for patients looking to advocacy groups for trusted information. Conclusion: Our experience with MTS shows that advocacy groups working together can create an important channel for engaging patients in learning about metastatic breast cancer trials. Next steps will be to expand our advocacy network, make it easier for users to narrow search results, and offer ways for patients to learn about newly opened clinical trials. Citation Format: Cohen EJ, Colen SL, Dahlke DV, Esser M, Flowers L, Guglielmino JE, Jenkins M, Knackmuhs G, Lusen R, Mertz SA, Esserman LJ. Metastatic trial search: Advocacy groups collaborate to engage metastatic breast cancer patients in clinical trials [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P2-14-01.