Objective: Observational studies can avoid biases by blinding medical records to characteristics of interest before outcome adjudication. However, blinding is costly. We assessed the effect of blinding race and geography on outcome ascertainment. Study Design and Setting: The Reasons for Geographic and Racial Differences in Stroke - Myocardial Infarction (REGARDS-MI) Study is an ancillary study to the REGARDS national prospective cohort study including 30,228 participants. The primary characteristics of interest are race and geography, and the prespecified acceptable agreement rate between adjudicators is set at less than 80%. We selected 116 suspected cardiovascular events that underwent adjudication with usual blinding. At least 3 months later, cases were readjudicated without blinding race and geographic location of the patient. We assessed differences in outcome ascertainment using Cohen's κ statistic and ARs. Results: Agreement between blinded and unblinded reviews was good to excellent for all four outcomes. κ statistics were 0.80 (chest pain), 0.85 (heart failure), 0.86 (revascularization), and 0.74 (MI) (P<0.0001 for all). Within each outcome, ARs were similar for race and geographic groups (agreement: 83-100%). Conclusion: In observational studies, blinding medical record review for outcome ascertainment for some types of patient characteristics may cause an unwarranted expense. © 2010 Elsevier Inc. All rights reserved.