Background Circadian rhythms regulate cellular physiology and could in fl uence the ef fi cacy of endocrine therapy (ET) in breast cancer (BC). We prospectively tested this hypothesis within the UNIRAD adjuvant phase III trial (NCT01805271). Methods 1278 patients with high-risk hormonal receptor positive (HR+)/ HER2 negative (HER2-) primary BC were randomly assigned to adjuvant ET with placebo or everolimus. Patients prospectively reported in a diary the daily timing of ET intake among four 6-h slots (06:00 - 11:59 (morning), 12:00 - 17:59 (afternoon), 18:00 - 23:59 (evening), or 24:00 - 05:59 (nighttime). The association between ET timing and disease-free survival (DFS) was a prespeci fi ed secondary endpoint of the trial and the results of this observational study are reported here. Findings ET timing was recorded by 855 patients (67.2%). Patients declaring morning (n = 465, 54.4%) or afternoon (n = 45, 5.4%) ET intake were older than those declaring evening (n = 339, 39.6%) or nighttime (n = 5, 0.6%) intake. With a median follow-up of 46.7 months, 118 patients had a local (n = 30) or metastasis relapse (n = 84), and 41 patients died. ET intake timing was not associated with DFS in the whole population (HR = 0.77, 95% CI [0.53 - 1.12]). The association between ET intake timing and DFS according to the strati fi cation factors revealed interactions with ET agent (tamoxifen versus Aromatase inhibitors (AI) with an increased DFS in the group of evening/nighttime versus morning/afternoon tamoxifen intake (HR = 0.43, 95% CI [0.22 - 0.85]), while no association was found for AI intake (HR = 1.07, 95% CI [0.68 - 1.69]). The interaction between ET intake timing and ET agent remained in multivariable analysis (HR = 0.38 [0.16 - 0.91]). Interpretation Tamoxifen intake in the evening/nighttime could be recommended in patients with high -risk HR+/ HER2- BC while awaiting for results from further ET timing studies. Funding UNIRAD was Supported by a grant from the French Ministry of Health PHRC 2012 and received funding from La Ligue contre le Cancer, Cancer Research -UK, Myriad Genetics, and Novartis. Copyright (c) 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY -NC -ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).