Background: Randomized controlled trials have shown a reduced risk of ischemic events and an increased risk of bleeding in patients treated with prolonged dual anti-platelet therapy (DAPT) beyond 12 months following acute coronary syndrome (ACS). We aimed to investigate outcomes of prolonged DAPT vs aspirin monotherapy (ASA) in a real-world population. Methods and results: Using nationwide registries, we identified all patients with ACS who underwent percutaneous coronary intervention and received 12-month DAPT between January 2013 and October 2016. Patients still on DAPT were compared to patients on ASA at index date (15 months after ACS-date) and followed for up to 2 years. Cox regression models were employed to calculate standardized risks of all-cause mortality, major adverse cardiovascular event (MACE), and major bleeding. The study included 7,449 patients, 1,901 on DAPT (median age 66, 72.1% male) and 5,548 on ASA (median age 65, 75.1% male). Standardized absolute 2-year risk of all-cause mortality, MACE, and major bleeding was 2.7%, 3.7%, and 5.4% for DAPT vs 2.2%, 3.8%, and 1.3% for ASA. DAPT was not associated with a significant standardized 2-year risk difference (SRD) of all-cause mortality (SRD: 0.5%, 95% confidence interval [CI]: -0.9 to 1.7) or MACE (SRD: -0.1%, 95% CI -1.8 to 1.6), but a significantly higher risk of major bleeding (SRD: 4.1%, 95% CI 1.8-6.6). Conclusions: In a nationwide cohort of ACS patients undergoing percutaneous coronary intervention, prolonged DAPT was not significantly associated with a reduced risk of all-cause mortality or MACE, but an increased risk of major bleeding. Future randomized controlled trials should investigate the optimal anti-platelet regimen in this patient group.