Background/Aims: Physicians’ perception of antecedent physical/cognitive status may account for the suboptimal implementation of acute coronary syndrome (ACS) guidelines. Methods: In an ACS survey of all cardiac wards, physicians’ perception of antecedent physical/cognitive status was prospectively recorded and categorized as either normal, mildly impaired or significantly impaired. We examined the impact of antecedent status on the use of evidence-based medications and procedures and on mortality. Results: Of the 2,021 patients, 1,025 (51%) had ST elevation. Impaired antecedent physical/cognitive status was diagnosed in 417 patients (20.6%), more commonly among non-ST-elevation patients (26.2 vs. 15.2%). Patients with impaired physical/cognitive status, with or without ST elevation, had significantly worse baseline demographic and clinical characteristics. They less often received aspirin, clopidogrel, platelet glycoprotein IIb/IIIa receptor antagonists, statins and β-adrenergic blockers, and significantly less often underwent in-hospital catheterization and revascularization. Reperfusion treatment was given significantly less frequently to ST elevation patients with impaired status (63.0% for normal vs. 50.8% and 33.3% for mildly and significantly impaired status, respectively; p = 0.001). After adjustment for differences in baseline characteristics, impaired antecedent status remained independently associated with lower use of these therapies and higher mortality rates. Conclusions: ACS guideline implementation is significantly influenced by physicians’ perception of antecedent physical/cognitive status, and thus is a crucial parameter for understanding ACS management and outcomes.