Scientific findings must withstand critical review if they are to be accepted as valid, and editorial peer review (critique, effort to disprove) is an essential element of the scientific process. We review the evidence of the editorial peer-review process of original research studies submitted for paper or electronic publication in biomedical journals. To estimate the effect of processes in editorial peer review. The following databases were searched to June 2004: CINAHL, Ovid, Cochrane Methodology Register, Dissertation abstracts, EMBASE, Evidence Based Medicine Reviews: ACP Journal Club, MEDLINE, PsycINFO, PubMed. We included prospective or retrospective comparative studies with two or more comparison groups, generated by random or other appropriate methods, and reporting original research, regardless of publication status. We hoped to find studies identifying good submissions on the basis of: importance of the topic dealt with, relevance of the topic to the journal, usefulness of the topic, soundness of methods, soundness of ethics, completeness and accuracy of reporting. Because of the diversity of study questions, viewpoints, methods, and outcomes, we carried out a descriptive review of included studies grouping them by broad study question. We included 28 studies. We found no clear-cut evidence of effect of the well-researched practice of reviewer and/or author concealment on the outcome of the quality assessment process (9 studies). Checklists and other standardisation media have some evidence to support their use (2 studies). There is no evidence that referees' training has any effect on the quality of the outcome (1 study). Different methods of communicating with reviewers and means of dissemination do not appear to have an effect on quality (3 studies). On the basis of one study, little can be said about the ability of the peer-review process to detect bias against unconventional drugs. Validity of peer review was tested by only one small study in a specialist area. Editorial peer review appears to make papers more readable and improve the general quality of reporting (2 studies), but the evidence for this has very limited generalisability. At present, little empirical evidence is available to support the use of editorial peer review as a mechanism to ensure quality of biomedical research. However, the methodological problems in studying peer review are many and complex. At present, the absence of evidence on efficacy and effectiveness cannot be interpreted as evidence of their absence. A large, well-funded programme of research on the effects of editorial peer review should be urgently launched.