Abstract Background: The primary purpose of this study was to evaluate the use of less-than-standard surgical therapy in early-stage operable breast cancer nationwide in Denmark. A secondary purpose was to evaluate the use primary endocrine therapy (PET). Methods: We assembled a nationwide, population-based cohort of early-stage operable breast cancer patients registered between 2000 and 2020 in Denmark using the Danish Breast Cancer Group Database, National Patient Registry, and National Pathology Registry. Patients were divided into two groups: those who underwent up-front surgery within 182 days of diagnosis and those who did not, with the latter group further subdivided by use of primary endocrine therapy versus not. We performed Cox regression analyses to assess all-cause mortality risk, adjusting for tumor size, age, comorbidity, estrogen receptor status and year of diagnosis. Results: We identified 84,487 patients with early-stage operable breast cancer. Of these 5,916 patients received less-than-standard therapy, and hereof 2,576 patients received PET. The number of patients for whom surgery was omitted increased from 226 (6.3%) in 2000 to 433 (11.8%) in 2020 and while none of the patients diagnosed in 2000 received PET 296 (8.1%) patients diagnosed in 2020 did. Less-than-standard therapy was associated with a significant higher rate of all-cause mortality. Conclusion: From 2000 to 2020 there was a significant increase in both omittance of surgery and the use of PET among breast cancer patients. Omitted surgery both with or without PET was associated with increased mortality, even after adjusting for tumor size, age, comorbidity, estrogen receptor status and year of diagnosis. Citation Format: Alan Celik, Tobias Berg, Maj-Britt Jensen, Ann Knoop, Bent Ejlertsen. Outcomes following less-than-standard therapy in operable breast cancer patients [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P3-10-04.