To measure the agreement between the newer European Society of Cardiology–American College of Cardiology (ESC-ACC) definition of acute myocardial infarction (AMI) and the traditional definition established by the World Health Organization (WHO).All adult ED patients admitted to our institution with at least one abnormally elevated cardiac biomarker were determined to have had an AMI by either or both definitions. The degree of agreement and the frequency of the reasons for disagreement between these 2 definitions were measured.A final study population consisted of 339 patients; 196 (58%) had an AMI by one or both definitions. Among them, 126 (64%; 95% confidence interval [CI], 57-71) were discordant for these 2 definitions: 104 (53%; 95% CI, 46-60) met only the ESC-ACC, whereas 22 (11%; 95% CI, 6-16) met only the WHO definition. Among those who met only the ESC-ACC definition, 37 (36%; 95% CI, 27-45) met none of the 3 traditional WHO criteria.More patients are discordant than concordant for the 2 standard definitions of AMI. Among them, a large majority meet only the new ESC-ACC definition.