Peer reviewers are blinded sometimes to authors' and institutions' names, but the effects of blinding on review quality are not known. We, therefore, conducted a randomized trial of blinded peer review. Each of 127 consecutive manuscripts of original research that were submitted to the Journal of General Internal Medicine were sent to two external reviewers, one of whom was randomly selected to receive a manuscript with the authors' and institutions' names removed. Reviewers were asked, but not required, to sign their reviews. Blinding was successful for 73% of reviewers. Quality of reviews was higher for the blinded manuscripts (3.5 vs 3.1 on a 5-point scale). Forty-three percent of reviewers signed their reviews, and blinding did not affect the proportion who signed. There was no association between signing and quality. Our study shows that, in our setting, blinding improves the quality of reviews and that research on the effects of peer review is possible.