Background: A strong association between on-thienopyridines platelet reactivity (PR) and the risk of both thrombotic and bleeding events in acute coronary syndrome (ACS) patients undergoing percutaneous coronary intervention (PCI) has been demonstrated. However, no study has analyzed the relationship between on-ticagrelor PR and clinical outcome in this clinical setting. Objectives: We aimed to investigate the relationship between on-ticagrelor PR, assessed by the vasodilator-stimulated phosphoprotein (VASP) index, and clinical outcome in patients with ACS undergoing PCI. Methods: We performed a prospective, multicenter, observational study of patients undergoing PCI for ACS. PR was measured using the VASP index following ticagrelor loading dose. The primary study endpoint was the rate of Bleeding Academic Research Consortium (BARC) type ≥2 at 1 year. The key secondary endpoint was the rate of major cardiovascular events (MACE) defined as the composite of cardiovascular death, myocardial infarction and urgent revascularization. Results: We included 570 ACS patients, among whom 33.9% had ST-elevation myocardial infarction. BARC type ≥ 2 bleeding occurred in 10.9% and MACE in 13.8%. PR was not associated with BARC ≥ 2 or with MACE (p=0.12 and p=0.56, respectively). No relationship between PR and outcomes was observed, neither when PR was analyzed quantitatively nor qualitatively (low on-treatment PR (LTPR) vs no LTPR). Conclusion: On-ticagrelor PR measured by the VASP was not associated with bleeding or thrombotic events in ACS patients undergoing PCI. PR measured by the VASP should not be used as a surrogate endpoint in studies on ticagrelor.