Abstract Background: Reproductive factors related to childbearing are recognized as breast cancer risk factors. Whether any of these factors could influence adult breast tissue composition is unclear. We investigated the associations of reproductive factors with percentage of epithelium, stroma, and fat tissue in benign breast biopsy samples. Methods: This study included 983 cancer-free women with biopsy-confirmed benign breast disease (BBD) within the Nurses’ Health Study and Nurses’ Health Study II cohorts. Percentage of each tissue type (epithelium stroma, and fat) was measured on whole section images with a deep-learning technique. All tissue measures were log-transformed in all the analyses to improve normality. The data on reproductive variables and other breast cancer risk factors were obtained from biennial questionnaires. Generalized linear regression was used to examine the associations of reproductive factors (parity, age at first birth, breastfeeding, age at menarche and the duration of the interval between menarche and age at first birth) with percentage of tissue types, while adjusting for known breast cancer risk factors.Results: In this study of 983 cancer-free women, 299 (30.4%) had non-proliferative disease, 559 (56.9%) had proliferative disease without atypia, and 125 (12.7%) had atypical hyperplasia, consistent with previously reported distributions of these BBD subtypes. The average proportion of epithelium, stroma, and fat in our study sample was 9.1% (range 0.5-52.2%), 72.4% (range 23.6-99.0%), and 18.5% (range 0-71.3%), respectively. As compared to parous women, nulliparous women had a smaller percentage of epithelium (β= -0.26, 95% confidence interval [CI] -0.41, -0.11) and fat (β= -0.34, 95% CI -0.54, -0.13) and a greater percentage of stroma (β=0.04, 95% CI 0.01, 0.08). Among parous women, number of children was inversely associated with percentage of stroma (β per child= -0.01 (-0.02, -0.00). Duration of breastfeeding of ≥24 months was associated with a reduced proportion of fat (β= -0.30, 95% CI -0.54, -0.06; p-trend=0.04). In a separate analysis restricted to premenopausal women, being nulliparous was associated with a greater proportion of stroma (β=0.06, 95% CI 0.02, 0.10) and smaller proportion of epithelium (β= -0.22, 95% CI -0.38, -0.06) and fat (β= -0.32, 95% CI -0.56, -0.08). Greater parity and older age at first birth were both associated with a greater proportion of epithelium (and a smaller proportion of stroma. The age at menarche and the duration of the interval between age at menarche and first birth were not associated with the proportion of any of the tissue types. Conclusions: Our findings suggest that reproductive factors with a protective effect on breast cancer risk may be associated with a greater proportion of epithelium and a smaller proportion of stroma, potentially suggesting importance of epithelial-stromal interactions. Future studies are warranted to confirm our findings and to elucidate the underlying biological mechanisms. Citation Format: Lusine Yaghjyan, Rebecca J Austin-Datta, Hannah Oh, Yujing J Heng, Adithya D Vellal, Korsuk Sirinukunwattana, Gabrielle M Baker, Laura Collins, Divya Murthy, Bernard Rosner, Rulla M Tamimi. Reproductive breast cancer risk factors and breast tissue composition on benign breast biopsies [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS7-90.