Introduction: Primary endocrine therapy (PET) is used variably in the UK as an alternative to surgery for older women with operable breast cancer. Guidelines state that only patients with “significant comorbidity” or “reduced life expectancy” should be treated this way and age should not be a factor. Methods: A Discrete Choice Experiment (DCE) was used to determine the impact of key variables (patient age, comorbidity, cognition, functional status, cancer stage, cancer biology) on healthcare professionals’ (HCP) treatment preferences for operable breast cancer among older women. Multinomial logistic regression was used to identify associations. Results: 40% (258/641) of questionnaires were returned. Five variables (age, co-morbidity, cognition, functional status and cancer size) independently demonstrated a significant association with treatment preference (p