A recent study suggested that results of single-center trials are frequently contradicted when similar trials are performed in multicenter settings. To perform a meta-epidemiologic study to evaluate whether estimates of treatment effect differ between single-center and multicenter randomized, controlled trials (RCTs). MEDLINE was searched via PubMed for meta-analyses of RCTs with binary outcomes that were published between August 2008 and January 2009 and in the first 6 months of 2010 in the 10 leading journals of each medical specialty. One issue of the Cochrane Database of Systematic Reviews was also searched. All individual RCTs included in the meta-analyses were selected. Data were extracted and their quality was assessed by use of the risk of bias tool of the Cochrane Collaboration. The primary outcome was the ratio of odds ratios (ROR), used to quantify the difference in estimated intervention effect between single-center and multicenter RCTs. An ROR less than 1 would indicate larger estimates of the intervention effect in single-center trials. Sensitivity analyses were performed with adjustment for sample size, risk of bias within RCTs, and variance of the log odds ratio to take publication bias into account. Forty-eight meta-analyses were selected, including 421 RCTs (223 were single-center and 198 were multicenter). Single-center RCTs showed a larger intervention effect than did multicenter RCTs (combined ROR, 0.73 [95% CI, 0.64 to 0.83]), with low heterogeneity across individual meta-analyses (I(2) = 12.0%; P = 0.24). Adjustment for sample size yielded consistent results (ROR, 0.85 [CI, 0.74 to 0.97]), as did adjustment for risk of bias within RCTs, such as allocation concealment (ROR, 0.76 [CI, 0.67 to 0.86]), and variance of log odds ratio (ROR, 0.83 [CI, 0.72 to 0.96]). Despite sensitivity analyses, meta-confounding cannot be fully excluded. Single-center RCTs showed larger treatment effects than did multicenter RCTs, a finding that was consistent in all sensitivity analyses. These results suggest that this item should be considered when the results of RCTs and meta-analyses are interpreted. Academic grant Recherche sur la Recherche from the Délégation Interrégionale à la Recherche Clinique (DIRC), Ile de France, Assistance Publique-Hôpitaux de Paris (APHP).