Abstract Background Breast cancer is the most common cancer among women, and tends to be more aggressive when diagnosed in women <40 years. However, limited research exists on breast health behaviors of women <40. This study analyzes trends in breast health behaviors and diagnostic data among women <40 served by the Avon Breast Health Outreach Program (BHOP) which links medically underserved women to breast health education and screening services. Methods Confidential client intake records of female clients (n= 192,296) of Avon BHOP grantees continuously-funded from 2011-2015 were analyzed, along with breast cancer diagnosis data from a select number of clients (n=82) from January-March 2016. Results Approximately 7% of Avon BHOP clients from 2011-2015 were <40 years (n=12,872). Women <40 had a significantly lower odds of having been taught breast self-exam (BSE), knowing about mammograms before their visit, having had a clinical breast exam (CBE) in the past 2 years, and having had a mammogram in the past 2 years compared to women ≥40. Over 80% of women <40 reported being taught BSE and knowing about mammograms before their visit, compared to 94% among women ≥40. The percent who reported a CBE in the past 2 years increased by approximately 5% among both <40 and ≥40 women, to a high of 64.6% and 78.2%, respectively, in 2015. Reporting a mammogram in the past 2 years also increased by approximately 5% in both groups, to a high of 25.9% and 70.3%, respectively, in 2015. Women <40 had a significantly higher odds of having breast-related symptoms (OR=7.64) and a relative diagnosed with breast cancer (OR=1.23) compared to women ≥40. For women <40, the adjusted odds of having had a mammogram in the past 2 years were 1.34 times higher (p<0.001) among those who had a relative diagnosed with breast cancer compared to those who did not, while the odds were 1.09 (p<0.001) among women ≥40. From January-March 2016, 10 breast cancer cases in women <40 were reported by 7 Avon BHOP grantees; 38 cases were reported in women ≥40. Of the 10 diagnosed cases, the majority were Hispanic (70%) and 30% were Stage IV compared to 6% among women ≥40, however the difference was not statistically significant (p=0.096). Discussion In 2015 the American Cancer Society estimated that 4.2% of breast cancer cases were diagnosed in women <40. While the Avon BHOP diagnosis data is limited, the results nonetheless demonstrate a higher rate of breast cancer in this age group. Additionally, the association between being <40 and having a later stage diagnosis compared to those ≥40 appears to be a meaningful trend, and, while not significant in our preliminary data, we anticipate the finding to persist. While Avon BHOP clients <40 had a significantly lower odds of knowing about screening behaviors compared to ≥40, a large percentage had knowledge of breast health, and increasingly reported higher rates of screening behaviors. There was also a greater positive association between having a relative with breast cancer and having had a mammogram in the past 2 years in women <40 compared to ≥40. These findings suggest that Avon BHOP clients <40 increasingly understand their breast cancer risk and seek screening services accordingly. It is important to provide clear screening guidelines for this population. Citation Format: Gates-Ferris K, Bauer KM, Senter L. Breast health behaviors of women under 40 accessing the Avon breast health outreach program [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 P2-07-10.