Summary. Background: Platelet hyperfunction contributes to acute coronary syndromes (ACS). Thus, we hypothesized that platelet function under high shear stress predicts recurrent ACS during long-term follow-up of ACS patients. Patients and methods: Consecutive ACS patients (n = 208) were prospectively followed-up for an average of 28 months. Platelet function was measured with the platelet function analyzer (PFA-100®; Dade Behring, Marburg, Germany) at baseline for collagen/adenosine diphosphate closure times (CADP-CT) and for collagen/epinephrine closure times (CEPI-CT) after infusion of a uniform dose of 250 mg aspirin. Results: Of the conventional risk factors, only the prevalence of diabetes was higher in ACS patients with re-events. However, use of clopidogrel and use of beta blockers were also slightly lower in patients with re-events (P < 0.05). The unadjusted risk hazard ratio (HR) for re-events was 3.3 [95% confidence interval (95% CI): 1.4–7.4; P = 0.005] in those patients with the shortest CADP-CT values (lowest quartile). Similarly, the risk was 2.0-fold higher (95% CI: 1.1–3.6; P = 0.02) in ACS patients with CEPI-CT < 300 s as compared with CEPI-CT ≥ 300 s. Inclusion of diabetes, clopidogrel and beta blockers in a multivariate Cox regression model enhanced the predictive value of CEPI-CT (HR: 2.7). Inclusion of von Willebrand factor levels did not alter the HR for recurrent ACS (HR: 2.1; 95% CI: 1.1–5.2; P = 0.03) for CEPI-CT < 300 s, but reduced the HR for CADP-CT (HR: 2.8, 95% CI: 0.8–9.8; P = 0.11). Conclusion: Shortened CT values reflect biologically relevant platelet hyperfunction in patients with ACS because they predict recurrent ACS.