Background— Stent thrombosis (ST) is an important end point in cardiovascular clinical trials. Adjudication is traditionally based on clinical event committee (CEC) review of case report forms and source documentation rather than angiograms. However, the degree to which this method of adjudication is concordant with the review of independent angiographic core laboratories (ACLs) has not been studied. This report represents the first assessment of variability between local investigators (LIs), a CEC, and an ACL. Methods and Results— Serial angiograms of 329 patients with acute coronary syndrome without ST-segment–elevation who underwent percutaneous coronary intervention at entry in the Trial to Assess the Effects of Vorapaxar in Preventing Heart Attack and Stroke in Particpants With Acute Coronary Syndrome (TRACER) and who met criteria for possible ST subsequent to the index event were reviewed by an ACL. The ACL was blinded to the assessment by both LIs and the CEC regarding the presence or absence of ST. CEC adjudication was based on Academic Research Consortium definitions of ST, using case report form data and source documents, including catheterization laboratory reports. The ACL, CEC, and LIs agreed on the presence or absence of ST in 52.9% events ( κ =0.32; 95% confidence interval, 0.26–0.39). The ACL and CEC agreed on 82.7% of events ( κ =0.57; 95% confidence interval, 0.47–0.67); the ACL and LIs agreed on 61.1% of events ( κ =0.25; 95% confidence interval, 0.16–0.34); and the CEC and LIs agreed on 62% of events ( κ =0.28; 95% confidence interval, 0.21–0.36). Conclusions— ST reporting by an ACL, a CEC, and LIs is discordant. The assessment of ST is more often detected by direct review of angiograms by an ACL. Clinical Trial Registration— URL: http://www.clinicaltrials.gov . Unique identifier: NCT00527943.