Objectives To investigate how often systematic reviewers encounter multiple trial effect estimates that are available for inclusion in a particular meta-analysis (multiplicity of results) and the methods they use to select effect estimates. Study Design and Setting We randomly sampled Cochrane and MEDLINE-indexed non-Cochrane reviews published between January 2010 and January 2012. The first presented meta-analysis of an effect measure for a continuous outcome in each review was identified, and methods to select results to include in this meta-analysis were extracted from review protocols and reviews. All effect estimates that were available for inclusion in the meta-analyses were extracted from trial reports. Results We examined 44 reviews. Multiplicity of results was common, occurring in 49 of trial reports (n = 210). Prespecification of decision rules to select results from multiple measurement scales and intervention/control groups (in multi-arm trials) was uncommon (19 and 14 of 21 review protocols, respectively). Overall, 70 of reviews included at least one randomized controlled trial with multiplicity of results, but this occurred less frequently in reviews with a protocol (risk difference, -25 ; 95 confidence interval: -52 , 1 ). Conclusion Systematic reviewers are likely to encounter multiplicity of results in the included trials. We recommend that systematic reviewers always consider predefining methods to select results to include in meta-analyses. Methods focusing on selection of measurement scales and how to deal with multi-arm trials would be most valuable.