OBJECTIVES: To describe the impact of peer reviewers on spin in reports of nonrandomized studies assessing a therapeutic intervention. STUDY DESIGN AND SETTING: This is a systematic review and retrospective before-after study. The sample consists of primary reports (n = 128) published in BioMed Central Medical Series journals between January 1, 2011, and December 31, 2013. The main outcome measures are the following: number and type of spin examples identified, deleted, or added by peer reviewers in the whole manuscript; number of reports with spin in abstract conclusions not detected by peer reviewers; the level of spin (i.e., no, low, moderate, and high level of spin) in the abstract conclusions before and after the peer review. RESULTS: For 70 (55%) submitted manuscripts, peer reviewers identified at least one example of spin. Of 123 unique examples of spin identified by peer reviewers, 82 (67%) were completely deleted by the authors. For 19 articles (15%), peer reviewers requested adding some spin, and for 11 (9%), the spin was added by the authors. Peer reviewers failed to identify spin in abstract conclusions of 97 (76%) reports. CONCLUSION: Peer reviewers identified many examples of spin in submitted manuscripts. However, their influence on changing spin in the abstract conclusions was low.