Abstract Background: Chemoprevention (including tamoxifen, raloxifene and exemestane) is a strategy to reduce breast cancer incidence in high risk women. Studies have shown at least a 50% decrease in the incidence of breast cancer in users of these drugs. The benefit is limited to a reduction in the incidence of ER/PR positive breast cancer. However, there is a growing population of women who have used these agents for primary breast cancer prevention, and a larger population of breast cancer survivors who have used these drugs as part of their systemic treatment. The purpose of this study was to identify a contemporary cohort of women with newly diagnosed breast cancers who had utilized chemoprevention strategies and describe their patterns of disease. Methods: The Breast Cancer Database at our institution was queried for patients who reported use of chemopreventive drugs and developed breast cancer between 1/2010-4/2014. Patients were divided into primary and secondary chemoprevention groups (no previous history of breast cancer and previous history of breast cancer, respectively). Descriptive statistics were utilized. Results: Out of a total of 1782 patients with newly diagnosed breast cancer, there were 106 (6%) patients who had used a chemopreventive agent. Out of these 106 patients, 91 (86%), had used the drug as part of their systemic therapy for prior breast cancer, with a median of 11 years from the initial diagnosis to the diagnosis of a second breast cancer. The primary chemoprevention group included women with risk based on family history and atypical hyperplasias. The majority of patients (81%) were diagnosed with early stage disease (stage 0, 1). In both groups, the majority of cancers were ductal in origin (84%). Eight of the 15 patients (53%) in the primary chemoprevention group were on treatment at the time of their cancer diagnosis; while 29% of patients in the secondary group were on treatment at the time of diagnosis. In the secondary group, 49% were contralateral second primary breast cancers and 44% were ipsilateral recurrences. Interestingly, the majority of cancers in both groups were ER/PR positive. Conclusions: Our cohort of women who used chemoprevention drugs were overwhelmingly diagnosed with early stage breast cancer, likely reflecting their commitment to screening. The majority of cancers were ER/PR positive. In this group, the choice of cancer treatment will need to be modified in light of prior hormonal treatment. Many of the patients in the secondary group were past users of prevention agents and further work is needed to clarify the duration of benefit of these drugs. In a similar vein, we look forward to research efforts to define the optimal age to initiate primary chemoprevention in high risk women. Citation Format: Jennifer Chun, Freya Schnabel, Shira Schwartz, Karen Hiotis, Amber Guth, Deborah Axelrod. History of chemoprevention in patients with newly diagnosed breast cancers [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P6-10-10.