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Background: It is not clear how the pathology, presentation and outcome for patients who present with de novo metastatic breast cancer (dnMBC) compare with those who later develop distant metastases. DnMBC is uncommon in younger patients. We describe these differences within a cohort of young patients in the United Kingdom., Methods: Women aged 40 years or younger with a first invasive breast cancer were recruited to the prospective POSH national cohort study. Baseline clinicopathological data were collected, with annual follow-up. Overall survival (OS) and post-distant relapse-free survival (PDRS) were assessed using Kaplan-Meier curves., Results: In total, 862 patients were diagnosed with metastatic disease. DnMBC prevalence was 2.6% (76/2977). Of those with initially localised disease, 27.1% (786/2901) subsequently developed a distant recurrence. Median follow-up was 11.00 years (95% CI 10.79-11.59). Patients who developed metastatic disease within 12 months had worse OS than dnMBC patients (HR 2.64; 1.84-3.77). For PDRS, dnMBC was better than all groups, including those who relapsed after 5 years. Of dnMBC patients, 1.3% had a gBRCA1, and 11.8% a gBRCA2 mutation., Conclusions: Young women with dnMBC have better PDRS than those who develop relapsed metastatic breast cancer. A gBRCA2 mutation was overrepresented in dnMBC.