Purpose: The placebo response rate in randomized controlled trials (RCTs) in ulcerative colitis (UC) varies from 0 to 76%. The aims of this study were to quantify the pooled placebo response rate and identify the factors affecting it. Methods: We performed a meta-analysis of 110 RCTs carried out between 1955 and 2005 and published in English. Regression analysis was used to identify factors significantly modifying placebo response. Results: The pooled placebo remission rate was 23% (95%CI: 18.4-28%) and the pooled placebo improvement rate was 32.1% (95%CI: 28.1-36.3%). Multivariate analysis showed that the country where the study was performed (P = 0.025 for placebo remission and P = 0.0083 for placebo response rates) significantly influenced the placebo remission and response rates. Conclusion: Placebo remission and response rates in RCTs of UC are highly variable and are significantly influenced by the country in which the RCT is performed. © 2007 Springer Science+Business Media, LLC.