Objectives: To see whether telling peer reviewers that their signed reviews of original research papers might be posted on the BMJ's website would affect the quality of their reviews.

Design: Randomised controlled trial.

Setting: A large international general medical journal based in the United Kingdom.

Participants: 541 authors, 471 peer reviewers, and 12 editors.

Intervention: Consecutive eligible papers were randomised either to have the reviewer's signed report made available on the BMJ's website alongside the published paper (intervention group) or to have the report made available only to the author-the BMJ's normal procedure (control group). The intervention was the act of revealing to reviewers-after they had agreed to review but before they undertook their review-that their signed report might appear on the website.

Main outcome measures: The main outcome measure was the quality of the reviews, as independently rated on a scale of 1 to 5 using a validated instrument by two editors and the corresponding author. Authors and editors were blind to the intervention group. Authors rated review quality before the fate of their paper had been decided. Additional outcomes were the time taken to complete the review and the reviewer's recommendation regarding publication.

Results: 558 manuscripts were randomised, and 471 manuscripts remained after exclusions. Of the 1039 reviewers approached to take part in the study, 568 (55%) declined. Two editors' evaluations of the quality of the peer review were obtained for all 471 manuscripts, with the corresponding author's evaluation obtained for 453. There was no significant difference in review quality between the intervention and control groups (mean difference for editors 0.04, 95% CI -0.09 to 0.17; for authors 0.06, 95% CI -0.09 to 0.20). Any possible difference in favour of the control group was well below the level regarded as editorially significant. Reviewers in the intervention group took significantly longer to review (mean difference 25 minutes, 95% CI 3.0 to 47.0 minutes).

Conclusion: Telling peer reviewers that their signed reviews might be available in the public domain on the BMJ's website had no important effect on review quality. Although the possibility of posting reviews online was associated with a high refusal rate among potential peer reviewers and an increase in the amount of time taken to write a review, we believe that the ethical arguments in favour of open peer review more than outweigh these disadvantages.