Purpose: The role of resection of the primary tumour in management of patients with OMBC is controversial. Loco-regional treatment (LRT) with curative intent could be discussed considered for OMBC patients, who experience good response to systemic therapy. We retrospectively studied the impact of resection of the primary tumour on overall survival (OS) for de novo OMBC treated at Oscar Lambret Center in Lille.Patients and methods: Between 2005 and 2017, all consecutive patients were selected. De novo OMBC was defined by 1 to 5 metastases in 1 or 2 organs, diagnosed within three months after primary tumor. Clinical data, tumor characteristics, metastatic sites, locoregional treatment, systemic first line treatment were recorded retrospectively. We set up 2 groups according to therapeutic strategy. Group 1 patients were managed with a prior planned curative intent surgery and systemic treatments and group 2 included patients with systemic therapy alone, without plan of surgery.Results: 116 patients were included in our analysis; 78 patients in group 1 and 38 patients in group 2. Median age was 54,9 years (25,2-86,2) with no difference between both groups (p=0,08). TNM stage, cancer histologies were comparable in both groups, excluding that there were more HER2+ tumour in Group 2 (p=0.003). 81 patients underwent LRT, 69 patients in group 1 and 12 patients in group 2. 59% of them had radical mastectomy and 89% had axillary surgery. For 29% of them, surgery was performed before knowledge of metastatic disease. The other main indications of breast surgery included stable disease (50%), local progression (9%), palliation (5%), single metastasis (5%) and complete metastatic response after chemotherapy (2%). 59 of 81 patients had adjuvant radiotherapy. Regarding systemic first line treatment, patients in group 1 received more chemotherapy (63% vs 86%, p=0.01), anti-HER2 treatment was appropriate except for 6 patients because of heart disease and 69% of patients had hormone therapy with no difference in both groups. Regarding metastases at diagnosis, the mean number of metastases was 2,1 (SD=1,2), 103 patients (89%) had one metastatic site, and 13 patients (11%) had two metastatic sites, with no difference between both groups. Secondary bone involvement was most prevalent site (69%). There was no difference between both groups in metastasis distribution. Median OS was 70,2 months in our cohort. LRT did not improve OS (HR 1,23; 95% CI [0,75-2,00] p=0,41), even after adjustment on age and HER2 status. Median progression free survival (PFS) was 30,2 months in our cohort. LRT did not improve PFS in comparison with systemic treatment alone (HR 1,38; 95% CI [0,91- 2,11] p=0,13).Conclusion: We found that LRT did not improve outcome (PFS or OS) in OMBC patients. This is in line with recently issued large randomized phase III trial. LRT is not a standard of care in OMBC, nevertheless, further clinical studies are needed to better identify the subgroup of OMBC patients that could benefit from LRT, at least in term of quality of life. Citation Format: Charlotte Debuquoy, Claudia Regis, Jennifer Wallet, David Pasquier, Nicolas Penel, Marie-Cecile Le Deley, Marie-Pierre Chauvet, Emilie Kaczmarek. The role of resection of the primary tumour in patients with de novo oligometastatic breast cancer (OMBC) [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS1-58.