CONTEXT: Editorial peer review is widely used to select submissions to journals for publication and is presumed to improve their usefulness. Sufficient research on peer review has been published to consider a synthesis of its effects. METHODS: To examine the evidence of the effects of editorial peer-review processes in biomedical journals, we conducted electronic and full-text searches of private and public databases to June 2000 and corresponded with the World Association of Medical Editors, European Association of Science Editors, Council of Science Editors, and researchers in the field to locate comparative studies assessing the effects of any stage of the peer-review process that made some attempt to control for confounding. Nineteen of 135 identified studies fulfilled our criteria. Because of the diversity of study questions, methods, and outcomes, we did not pool results. RESULTS: Nine studies considered the effects of concealing reviewer/author identity. Four studies suggested that concealing reviewer or author identity affected review quality (mostly positively); however, methodological limitations make their findings ambiguous, and other studies' results were either negative or inconclusive. One study suggested that a statistical checklist can improve report quality, but another failed to find an effect of publishing another checklist. One study found no evidence that training referees improves performance and another showed increased interrater reliability; both used open designs, making interpretation difficult. Two studies of how journals communicate with reviewers did not demonstrate any effect on review quality. One study failed to show reviewer bias, but the findings may not be generalizable. One nonrandomized study compared the quality of articles published in peer-reviewed vs other journals. Two studies showed that editorial processes make articles more readable and improve the quality of reporting, but the findings may have limited generalizability to other journals. CONCLUSIONS: Editorial peer review, although widely used, is largely untested and its effects are uncertain.