Background:Indications for adjuvant endocrine treatment of breast cancer have gradually increased over the past several years. We aimed to define subgroups of patients who may or may not benefit from adjuvant endocrine therapy. Methods:A population-based cohort of systemically untreated breast cancer patients (N = 3197) were identified within the registry of the Danish Breast Cancer Cooperative Group (DBCG). The patients were node negative and had estrogen receptor-positive and/or progesterone receptor-positive tumors (except medullary tumors) and were further characterized by the following risk factors: aged 35-74 years (grouped into 5-year categories) at surgery, tumor size (≤20 mm), and histopathology (grade 1 ductal carcinoma, grade 1 or 2 invasive lobular carcinoma, other or unknown histopathology). Standardized mortality ratios (SMRs) were calculated based on the mortality rate (observed number of deaths per 100 000 person-years) among patients relative to the mortality rate in the general population of women (expected number of deaths per 100 000 person-years). The association between standardized mortality ratio and risk factors were analyzed in univariate and multivariable Poisson regression models. All findings were validated in a subsequent DBCG cohort of breast cancer patients (N = 2710). Results:The median follow-up after surgery was 14.8 years. In the study population there were 970 deaths compared with expected death of 737 women, which was an excess mortality of 233 deaths (SMR = 1.32, 95% CI = 1.24 to 1.40). Mortality rates were 2356 per 100 000 person-years in the study population and 1790 per 100 000 person-years in the general population of women. The mortality rate was associated with larger tumor size (11-20 mm tumors vs 1-10 mm tumors, SMR = 1.42, 95% confidence interval [CI] = 1.31 to 1.53 vs SMR = 1.12, 95% CI = 1.00 to 1.26). The mortality rate was also associated with age (35-59 years, SMR > 1) compared with that in the general population of age-matched women, except for a small subgroup of patients (aged 60-74 years, tumors ≤10 mm, grade 1 ductal carcinoma, and grade 1 or 2 lobular carcinoma: adjusted relative risk = 1.02, 95% CI = 0.89 to 1.16.). Conclusions:A small subgroup of breast cancer patients who were 60 years or older and had hormone-responsive earlystage tumors up to 10 mm, and received no systemic adjuvant therapy, were not at increased risk of mortality compared with women in this age-group in the general population.