BACKGROUND: Older women have lower breast cancer surgery rates than younger women. UK policy states that differences in cancer treatment by age can only be justified by patient choice or poor health. METHODS: We investigate whether lack of surgery for older patients is explained by patient choice/poor health in a prospective cohort study of 800 women aged ≥70 years diagnosed with operable (stage 1-3a) breast cancer at 22 English breast cancer units in 2010-2013. DATA COLLECTION: interviews and case note review. OUTCOME MEASURE: surgery for operable (stage 1-3a) breast cancer <90 days of diagnosis. Logistic regression adjusts for age, health measures, tumour characteristics, socio-demographics and patient's/surgeon's perceived responsibility for treatment decisions. RESULTS: In the univariable analyses, increasing age predicts not undergoing surgery from the age of 75 years, compared with 70-74-year-olds. Adjusting for health measures and choice, only women aged ≥85 years have reduced odds of surgery (OR 0.18, 95% CI: 0.07-0.44). Each point increase in Activities of Daily Living score (worsening functional status) reduced the odds of surgery by over a fifth (OR 0.23, 95% CI: 0.15-0.35). Patient's role in the treatment decisions made no difference to whether they received surgery or not; those who were active/collaborative were as likely to get surgery as those who were passive, that is, left the decision up to the surgeon. CONCLUSION: Lower surgery rates, among older women with breast cancer, are unlikely to be due to patients actively opting out of having this treatment. However, poorer health explains the difference in surgery between 75-84-year-olds and younger women. Lack of surgery for women aged ≥85 years persists even when health and patient choice are adjusted for.