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Background: Tumor progression following endocrine therapy is considered to indicate resistance to endocrine drugs due to a variety of mechanisms. An insufficient dose of endocrine drugs is one of the causes for treatment failure in some patients with high hormone-receptor (HR)-expressing advanced breast cancer. This study aimed to explore the efficacy of high-dose tamoxifen (TAM) treatment in patients with advanced breast cancer with highly expressed HR., Materials & Methods: This was a single-arm, phase II pilot study that enrolled patients with advanced breast cancer with high HR expression (estrogen receptor ⩾60% and/or progesterone receptor ⩾60%) following routine endocrine therapy. All enrolled patients received a high-dose of TAM (100 mg/day) until disease progression. The primary endpoint was progression-free survival (PFS). The secondary endpoints included objective response rate (ORR), clinical benefit rate (CBR), overall survival (OS), and safety. Exploratory endpoints included the predictive value of 16 α- 18 F- 17 β-fluoroestradiol quantitative positron emission tomography/computed tomography ( 18 F-FES PET/CT) for treatment efficacy., Results: A total of 30 patients were enrolled between September 2017 and February 2019. The median PFS was 6 months [95% confidence interval (CI) 4.9-7.1] and the median OS was 15.6 months (95% CI 8.3-22.9). Five patients experienced a partial response (PR) and none experienced a complete response (CR), with an ORR of 16.7% and CBR of 33.3%. No severe adverse events were observed. Lesions with 18 F-FES maximum standardized uptake value (SUVmax) ⩾4 had a significantly longer PFS [median 9.2 months, (95% CI 6.9-11.6)] compared with lesions with a 18 F-FES SUVmax <4 [median 4.8 months, (95% CI 3.9-5.6); p  = 0.022]., Conclusion: A high-dose of TAM is effective and safe for patients with advanced breast cancer with high HR expression. 18 F-FES SUVmax values may predict the local clinical benefits of high-dose TAM ., Trial Registration: [ClinicalTrials.gov identifier: NCT0304565]., Competing Interests: Conflict of interest statement: The authors declare that there is no conflict of interest., (© The Author(s), 2021.)