OBJECTIVES Emergency physician threshold to test for acute coronary syndrome (ACS) is directly related to ACS diagnosis rate and inversely related to ACS missed diagnosis rate. Feedback to emergency physicians of information on their prior patients whose ACS diagnosis was not identified may improve physician diagnostic performance. METHODS A critical pathway for evaluation of patients for ACS was modified to include feedback to physicians on their cases who had a return visit and did not have their ACS diagnosis identified at their prior emergency department visit. Feedback included case-specific details, discussion of the case at the monthly Morbidity and Mortality conference, and a yearly a report to each physician comparing their performance to their peers (ACS evaluation rate, ACS diagnosis rate, and ACS missed diagnosis rate). Cases were identified, and physician-specific performance was calculated from a computerized encounter database at 2 community teaching hospitals. RESULTS During the study period, 29 emergency physicians evaluated 295,758 patients and identified 6472 ACS cases. During the study, the yearly ACS evaluation rate for individual physician ranged from 19% to 70% (average 40.3%; 95% confidence interval [CI], 39.5%-41.1%), the yearly ACS diagnosis rate for individual physician ranged from 1.1% to 4.2% (average 1.7%; 95% CI, 1.65%-1.75%), and the yearly missed ACS diagnosis rate for individual physician ranged from 0% to 17% (average 2.8%; 95% CI, 2.3%-3.3%). Individual physician ACS evaluation rate was directly related to physician ACS diagnosis rate (r 0.76, P = 0.00012) and was inversely related to that physician missed ACS rate (r 0.45, P = 0.001). During the study, implementation of the critical pathway increased the ACS evaluation rate from 30% to 48% and decreased the ACS missed diagnosis rate from 1.5% to 0.3%. CONCLUSIONS Emergency physicians with lower threshold for ACS evaluation more frequently diagnose patients with ACS and less frequently miss the diagnosis of ACS. Feedback to emergency physicians of information on their patient's return visits and their own diagnostic performance may improve outcome for patients with ACS.