Although several risk factors exist for acute coronary syndrome (ACS) no biomarkers for survival or risk of re-infarction have been validated. Previously, reduced serum concentrations of anti-ß1AR Ab have been implicated in poorer ACS outcomes. This study further evaluates the prognostic implications of anti-ß1AR-Ab levels at the time of ACS onset. Serum anti-ß1AR Ab concentrations were measured in randomly selected patients from within the PLATO cohort. Stratification was performed according to ACS event: ST-elevation myocardial infarct (STEMI) vs. non-ST elevation myocardial infarct (NSTEMI). Antibody concentrations at ACS presentation were compared to 12-month all-cause and cardiovascular mortality, as well as 12-month re-infarction. Sub-analysis, stratifying for age and the correlation between antibody concentration and conventional cardiac risk-factors was subsequently performed. Serum anti-ß1AR Ab concentrations were measured in 400/799 (50%) STEMI patients and 399 NSTEMI patients. Increasing anti-ß1AR Ab concentrations were associated with STEMI (p = 0.001). Across all ACS patients, no associations between anti-ß1AR Ab concentration and either all-cause cardiovascular death or myocardial re-infarction (p = 0.14) were evident. However among STEMI patients ≤60 years with anti-ß1AR Ab concentration <median higher rates of re-infarction were observed, compared to those with anti-ß1AR Ab concentrations > median (14/198 (7.1%) vs. 2/190 (1.1%)); p = 0.01). Similarly, the same sub-group demonstrated greater risk of cardiovascular death in year 1, including re-infarction and stroke (22/198 (11.1%) vs. 10/190 (5.3%); p = 0.017). ACS Patients ≤60 years, exhibiting lower concentrations of ß1AR Ab carry a greater risk for early re-infarction and cardiovascular death. Large, prospective studies quantitatively assessing the prognostic relevance of Anti-ß1AR Ab levels should be considered.