We evaluated the impact of using the new universal definition of myocardial infarction (MI) criteria implemented with a 20% increment between 2 cardiac troponin I (cTnI) measurements. The study included 284 consecutive episodes of patients admitted to the emergency department with suspected acute coronary syndrome (ACS) and an initial cTnI measurement of 0.10 ng/mL (0.10 microg/L) or less followed by 1 or more measurements within 24 hours. Episodes with a maximum cTnI above the 99th percentile (0.04 ng/mL [0.04 microg/L]) and a dynamic increase between 2 measurements of 20% or more were considered to meet MI criteria. Of the 284 episodes, 109 (38.4%) had a maximum cTnI higher than 0.04 ng/mL (0.04 microg/L). However, only 66 episodes (23.2%) also had an increase of 20% or more in the cTnI concentration and met MI criteria. These 66 episodes included 37 patients diagnosed with an MI and 29 patients not diagnosed with an MI. The 29 patients who also met MI criteria were more frequently readmitted for ACS within 6 months.