Objective: Up to 40% of women over 70 years with primary operable breast cancer in the UK are treated with primary endocrine therapy(PET) as an alternative to surgery. A variety of factors are important in determining treatment for older breast cancer patients. This study aimed to identify the patient and tumor factors associated with treatment allocation in this population.Methods: Prospectively collected data on treatment received(surgery vs. PET) were analysed with multivariable logistic regression using the variables age, modified Charlson Comorbidity Index(CCI), activities of daily living(ADL) score, Mini-Mental State Examination(MMSE) score, HER2 status, tumour size, grade and nodal status. Results: Data were available for 1,122 cancers in 1,098 patients recruited between February 2013 and June 2015 from 51 UK hospitals. About 78% of the population were treated surgically, with the remainder being treated with PET. Increasing patient age at diagnosis, increasing CCI score, large tumor size(5 cm or more) and dependence in one or more ADL categories were all strongly associated with non-surgical treatment(P<0.05).Conclusion: Increasing comorbidity, large tumor size and reduced functional ability are associated with reduced likelihood of surgical treatment of breast cancer in older patients. However, age itself remains a significant factor for non-surgical treatment; reinforcing the need for evidence-based guidelines. Objective: Up to 40% of women over the age of 70 years with primary operable breast cancer in the UK are treated with primary endocrine therapy (PET) as an alternative to surgery. A variety of factors are important in determining treatment for older breast cancer patients. studied aimed to identify the patient and tumor factors associated with treatment allocation in this population. Methods: Prospectively collected data on treatment received (surgery vs. PET) were analyzed with multivariable logistic regression using the variables age, modified Charlson Comorbidity Index (CCI), activities of daily living (ADL) score, Mini-Mental State Examination (MMSE) score, HER2 status, size size, grade and nodal status. Results: Data were available for 1,122 cancers in 1,098 patients recruited between February 2013 and June 2015 from 51 UK hospitals. About 78% of the population were treated surgically, with the remainder of treated with PET. Increasing patient age at diagnosis, increasing CCI score, large tumor size (5 cm or more) and dependence in one or more ADL categories were all strongly associated with non-surgical treatment (P <0.05) .Conlusion: Increasing comorbidity, large tumor size and reduced functional ability are associated with reduced likelihood of surgical Treatment of breast cancer in older patients. However, age itself remains a significant factor for non-surgical treatment; reinforcing the need for evidence-based guidelines.