Background: The Clinical Treatment Score at 5 years (CTS5) is a prognostic tool using clinicopathological data to estimate distant recurrence (DR) risk after 5 years of endocrine therapy for postmenopausal women with estrogen receptor positive (ER+) breast cancer. It was developed and validated in the ATAC and BIG 1-98 trials. Methods: The validity of CTS5 was tested in a retrospective cohort of unselected, non-trial patients diagnosed with early ER+ breast cancer at the Royal Marsden Hospital from 2000-2007 who were alive and distant recurrence-free at 5 years. The primary endpoint was time to late DR (5-10 years). Cox regression models were used to determine the prognostic value of CTS5 and to produce Kaplan-Meier curves with associated 10-year DR risks (%). Results: A total of 2428 women were included with a median follow-up of 9.34 years from diagnosis. The CTS5 was significantly prognostic for late DR in post- and premenopausal women (Table). Risk stratification by CTS5 of the postmenopausal cohort was comparable with the development cohort. 42.1% of postmenopausal women were categorised into the low risk group with a late DR risk of 4.9% and these women had significantly lower risk of late DR compared to those in the intermediate or high-risk groups (Table). Amongst the premenopausal cohort, 41% were categorised as low risk with a late DR risk of 4.9%. The prognostic effect of CTS5 was seen for chemotherapy treated (HR=2.26, 95% CI (1.68-3.03)) and untreated patients (HR=1.93, 95% (1.32-2.82)). Conclusions: CTS5 demonstrated clinical validity for predicting late DR within a large cohort of unselected, non-trial patients that included premenopausal women. The low risk cohort identified by the CTS5 represents a group of women whose risk of late DR is so low as to not warrant extended endocrine therapy to ten years.