We prospectively and blindly assessed the diagnostic and prognostic impact of implementation of the European Society of Cardiology/American College of Cardiology recommendations for redefinition of myocardial infarction (MI) in an unselected cohort of patients with suspected cardiac chest pain, with particular attention to prespecified clinical groups. All patients admitted to our institute with suspected cardiac chest pain were enrolled. Physicians provided usual care using serial electrocardiograms/creatine kinase (CK)/aspartate transaminase according to World Health Organization (WHO) criteria for MI, while blinded to additional measurements of cardiac troponin T (cTnT) and CK-MB mass. After discharge, diagnoses based on WHO and new criteria were compared, and major adverse cardiac events monitored for 6 months. Implementation of the new recommendations classified an additional 26.1% of patients as having MI compared with WHO criteria, and produced an overall diagnostic alteration in 11.5%. Two thirds of the additional patients with MI were previously diagnosed with unstable angina, whereas one third had "other cardiac" or "noncardiac" diagnoses. A similar MI cohort to the cTnT diagnosis was identified using a CK-MB mass discriminator value of 5 microg/L, but not 10 microg/L. The 6-month prognosis was similar in patients diagnosed with MI by new (cTnT) and WHO criteria, with the new criteria thus identifying a further high-risk cohort in the WHO negative group. In our cohort, the new Joint European Society of Cardiology/American College of Cardiology recommendations identify one fourth more patients as having MI. The 6-month prognosis of those patients reclassified as having MI was similar to those diagnosed with MI by both criteria.