Risk stratification of chest pain (CP) is still debated. Objective of this study was to evaluate the performance of a risk stratification model for patients with suspected acute coronary syndrome (ACS) and nondiagnostic ECG at presentation, in whom the occurrence of myocardial infarction was either diagnosed following traditional (t-MI) or the recently redefined (r-MI) criteria. First-line 6-h work-up categorized 3068 patients with suspected ACS and nondiagnostic ECG into low-risk for short-term coronary events, intermediate-risk who entered second-line work-up, and high-risk. Intermediate-risk patients with positive second-line work-up and high-risk patients were considered for urgent coronary angiography. Angina, non-fatal MI, sudden death, and revascularization constituted composite end-point (CE) for in-hospital and 6-month outcome. ACS incidence was 16%; r-MI increased by 62% the diagnosis of MI over t-MI. Among 2024 discharged low-risk patients, 12 (0.6%) had non-fatal CE at 6 months. ACS was diagnosed in 19% of 503 intermediate-risk and 96% of 389 high-risk patients. Among ACS patients, in-hospital CE occurred in 14% of t-MI, 7% of r-MI, and 9% of unstable angina (UA) patients (t-MI vs. r-MI and t-MI vs. UA: p<0.05, for both); 6-month CE occurred in 23%, 16% and 12% of t-MI, r-MI and UA, respectively (t-MI vs. UA: p<005). Sensitivity, specificity and accuracy were high both for diagnostic (97%, 98%, 99%, respectively) and treatment (83%, 98%, 97%, respectively) strategy. Risk stratification, and categorization according to traditional or redefined MI and UA criteria allow safe allocation of resources in CP patients with suspected ACS and nondiagnostic ECG at presentation because outcome is accurately predicted.