Background: Guidelines list more than 20 regimens as options for adjuvant breast cancer treatment. Adjuvant!Online provides risk assessment based on patient characteristics and expected risk reduction, but lacks guidance on specific treatment choice. An online tool was developed to provide expert guidance in choice of adj TX for EBC. Methods: Six experts made first and second treatment recommendations for 864 patient case variations based on age, menopausal status, nodes, tumor grade/size, and receptor status (HR, HER2). Their choices were incorporated into an interactive decision support tool. Users of the tool entered specific patient characteristics and their own choice of therapy; then, expert recommendations were displayed for a patient case variation matching the user’s case, and users were asked if those recommendations impacted their treatment decision. Expert and participant responses were analyzed. Results: In cases needing adj TX, experts all chose tamoxifen for premenopausal HR+, AI for postmenopausal HR+ cases, and a trastuzumab-containing regimen for HER2+ cases. Experts varied in their choice of chemotx in HER2- (AC-P wkly, 37%; DCx4, 30%; DAC, 21%; AC-P dd, 8%; and CMF, 4%) and HER2+ cases (DCbH, 64%; AC-PH, 34%; and other-H, 3%). More than 2000 users accessed the tool, including 1043 physicians. When prompted, “Did the tool impact your adjuvant treatment decision?” 65% of physicians (n = 669) said “yes” for at least one case. Cases entered by users appear to reflect incidence of patient types in the general population. Participants’ initial treatment choices included some regimens never selected by experts with a different frequency for chemotx in HER2- (AC-P q3wk, 20%; DC, 17%; AC, 16%; AC-P wkly, 12%; FEC, 8%; CMF, 7%; FEC-D, 6%; AC-P dd, 5%; DAC, 3%; AC-D, 3%) and trastuzumab+chemo in HER2+ (AC-PH, 38%; DCbH, 27%; other-H, 17%; AC-DH, 14%; and DH-FEC, 3%). Detailed analysis by specific characteristics will be presented. Conclusions: Tools that provide expert guidance on adjuvant treatment of breast cancer may impact clinicians’ treatment decisions and improve patient care. Supported by education grants from Genentech and Pfizer.