The purpose of the study was to examine adherence to hormone therapy (HT) in elderly breast cancer patients (≥ 65 years old) treated with hypofractionated radiotherapy. We analyzed data on 550 ER-positive breast cancer patients given hypofractionated whole-breast radiotherapy from June 2009 to September 2016. Baseline comorbidities considered in the hypertension-augmented Charlson Comorbidity Index (hCCI) were retrospectively retrieved. Total hCCI scores were classified as no comorbidity (hCCI = 0), low burden of comorbidity (hCCI = 1), and high burden of comorbidity (hCCI ≥ 2). Competing risk analysis was used to estimate the 5-year cumulative incidence of HT discontinuation. Fine and Gray models were used to estimate the adjusted subhazard ratio (SHR) of HT discontinuation by hCCI score. HT was initially prescribed for 85.6% of patients and almost all of them (468/471) took it for at least one month. It was subsequently discontinued by 45 patients (9.6%), for an overall 5-year cumulative incidence of 11.7%. The 5-year cumulative incidence of HT discontinuation rose from 3.9% in the youngest age group (65-69 years) to 23.3% in the oldest (≥ 80 years) (p = 0.005). Baseline comorbidity had some effect on the likelihood of discontinuing HT, but only among patients with a low burden of comorbidity (hCCI = 1, SHR 2.00, 95%CI 0.95-4.20). Adherence to HT was better in our sample than in the literature, probably because patients were selected and motivated to continue HT. This confirms the importance of communication with patients to improve adherence to HT. We confirmed the association between HT discontinuation and older age, while comorbidity had a limited influence.