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Background and Aims: Perianal fistulizing disease is a common complication of Crohn's disease [CD], for which new therapies are urgently needed. To assist the design of clinical trials for novel therapeutics, we conducted a systematic review and meta-analysis of randomised controlled trials [RCTs] to quantify placebo rates and identify factors influencing them in perianal CD [pCD]., Methods: We searched MEDLINE, Embase and CENTRAL from inception to June 2021. Eligible studies were placebo-controlled trials of pharmacological interventions for pCD. Placebo fistula response and remission rates for induction and maintenance trials were extracted and pooled using a random-effects model. Mixed-effects meta-regression was used to evaluate the impact of patient and study-level characteristics on point estimates., Results: In 17 RCTs [13 induction, five maintenance] the pooled placebo fistula response and remission rate for induction trials was 25% (95% confidence interval [CI] 17-36%) and 17% [95% CI 11-25%], respectively. For maintenance trials, the pooled placebo fistula response and remission rate was 23% [95% CI 17-32%] and 19% [95% CI 14-25%], respectively. Trials enrolling patients with less disease activity and a higher proportion with ileal predominant disease were associated with significantly higher placebo response rates. Trials originating in Europe [compared to North America], therapies requiring perianal injection and a longer timepoint to measure remission were associated with higher placebo remission rates., Conclusions: Placebo response and remission rates in pCD trials are influenced by patient and disease-related factors, as well as the type of intervention being studied. These contemporary rates will inform trial design for novel therapeutics., (© The Author(s) 2022. Published by Oxford University Press on behalf of European Crohn’s and Colitis Organisation. All rights reserved. For permissions, please email: journals.permissions@oup.com.)