PURPOSE: Endocrine therapy (ET) is the mainstream adjuvant treatment for ER-positive breast cancer (BC). We analysed 9293 ER-positive BC patients diagnosed in nine European countries in 2009-2013 to investigate how comorbidities at diagnosis, age, stage and subtype affected ET use over time, and relapse. METHODS: Adjusted odds ratios (ORs) and 9595 of receiving ET were estimated according to Charlson comorbidity, age, stage and subtype using logistic regression. The 2-year cumulative incidence and adjusted sub-hazard ratios (SHRs) of relapse were estimated using competing risk analysis, with all-cause death as the competing event. The z-test was used to assess differences in the proportion of patients receiving ET in 1996-1998 and 2009-2013. RESULTS: Ninety percent of the patients started adjuvant ET, range 96Belgium, Estonia, Slovenia, Spain)-75Switzerland). ORs of starting ET were lower for women aged textgreater 75 years, with severe comorbidities, or luminal B HER2-positive cancer. The factors independently increasing the risk of relapse were: not receiving ET (SHR 2.26, 95.02-5.03); severe comorbidity (SHR 1.94, 95.06-3.55); luminal B, either HER2 negative (SHR 3.06, 95.61-5.79) or positive (SHR 3.10, 95.36-7.07); stage II (SHR 3.20, 95.56-6.57) or stage III (SHR 7.41, 95.48-15.73). ET use increased significantly but differently across countries from 51-85996-1998 to 86-96009-2013. CONCLUSIONS: ER-positive BC patients in Europe are increasingly prescribed ET but between-country disparities persist. Older women and women with severe comorbidity less frequently receive ET. ET omission and severe comorbidity independently predict early disease relapse.