Abstract Background Regular mammographic screening can reduce breast cancer morbidity and mortality. Participation rates are suboptimal in Australia’s fully funded biennial breastscreening program (BreastScreen) for women aged 50-74. Despite obesity being a well-established risk factor for post-menopausal breast cancer, cross sectional data suggests obesity may be a risk factor for non-participation in recommended screening, due to adverse screening experiences. This research aimed to ascertain the link obesity and non-participation by using data linkage of routinely collected data. Methods Data for women age eligible for breast screening were linked between the NSW Cancer Registry and the Australian Longitudinal Study of Women’s Health (ALSWH) to create a cohort of women who either participated in screening as recommended or not. Women from the 1946-1951 ALSWH birth cohort were included in the study. These women reported BMI via 8 survey waves. The primary outcome was adherence to breast screening measured by frequency of screening over the follow-up period (1998-2016). Unadjusted risk ratios were calculated using mixed-effects logistic regression for the association between BMI and screening participation. Results The study included 2804 linked records of age eligible women (mean age of 52.37[SD 5.47]). 22.8% of the cohort were obese (BMI>30kg/m2). Obesity was significantly associated with non-recommended screening participation (screening within 3 years of last breast screen); odds ratio 1.63 (95% confidence interval 1.32 to 2.00, p < 0.0001). Conclusions Obesity has a significantly impact on recommended participation in a nationally provided breast screening program, despite obesity being a risk factor for post- menopausal breast cancer. Optimising participation among higher risk and under-screened women in under utilised breast cancer screening programs is warranted. Development of targeted interventions to increase screening participation among these higher risk women is needed. Key messages • Women living with obesity and less likely to participate in recommended breast screening. • Targeted interventions are needed to optimise participation in breast screening to ensure these higher risk women are not at higher risk of adverse outcomes due to breast cancer.