Objectives The Manchester Acute Coronary Syndromes (MACS) decision rule has been shown to be a powerful diagnostic tool in emergency department (ED) patients with suspected acute coronary syndromes (ACS). It has the potential to improve system efficiency by identifying patients suitable for discharge after a single blood draw for high-sensitivity troponin and heart-type fatty acid-binding protein (h-FABP) analysis at presentation to the ED. The objective was to externally validate the MACS decision rule and establish its diagnostic accuracy as a discharge tool in a new set of prospectively recruited ED patients. Methods In this preplanned analysis of a prospectively recruited single-center cohort, consecutive ED patients ≥18 years with suspected ACS were included. Testing for h-FABP and high-sensitivity troponin T was undertaken on serum drawn on arrival, and any clinical features required to calculate the MACS rule were recorded. The primary outcome was major adverse cardiac events (MACE) within 30 days (acute myocardial infarction [AMI], death, or revascularization). The secondary outcome was AMI alone, adjudicated using 6-hour troponin results. Results Of the 782 participants included, 78 (10.0%) developed MACE and 61 (7.8%) had an AMI. Of participants, 133 (17.0%) were identified as "very low risk" and therefore suitable for immediate discharge with a 0% incidence of MACE or AMI. Of remaining patients, 314 (40.2%) were "low risk," 320 (40.9%) were "moderate risk," and 15 (1.9%) were "high risk," with incidences of MACE of 2.2, 19.7, and 53.3%, respectively. The sensitivity was 100% (95% confidence interval [CI] = 95.4% to 100%) for MACE at 30 days and 100% (95% CI = 94.1% to 100%) for AMI. The area under the receiver operating characteristic curve was 0.87 (95% CI = 0.83 to 0.91) for the MACS rule in the prediction of MACE. Conclusions In this prospectively recruited cohort of ED patients with suspected ACS, the MACS decision rule identifies a significant proportion of patients who are suitable for immediate discharge after a single blood draw at presentation, with a very low risk of MACE at 30 days. This study externally validates previous findings that the MACS rule is a powerful diagnostic tool in this setting. A randomized controlled trial to establish the utility of the rule in an everyday clinical setting is justified.