To the Editor: The selective reporting of outcomes in randomized controlled trials (RCTs), or outcome reporting bias, has been identified,1 but its prevalence and potential effect in meta-analyses is unknown. We investigated (1) to what extent Cochrane meta-analyses include only a proportion of identified RCTs when estimating treatment effects on patient-important outcomes and (2) whether the proportion of RCTs included in a meta-analysis is associated with its pooled effect size.