SummaryBackgroundCompulsory community treatment (CCT) aims to reduce hospital readmissions among people with mental illness. However, research examining the usefulness of CCT is inconclusive. We aimed to assess the effectiveness of CCT in reducing readmission and length of stay in hospital and increasing community service use and treatment adherence.MethodsFor this systematic review and meta-analysis, we searched three databases (PsycINFO, MEDLINE and Embase) for quantitative studies on CCT published in English between Jan 1, 1806, and Jan 4, 2018. We included both randomised and non-randomised designs that compared CCT with no CCT, and pre-post designs that compared patients before and after CCT. Studies were eligible if they had been peer-reviewed, if 50% or more of patients had severe mental illness, and if CCT was the intervention. Trials in which CCT was used in response to a criminal offence were excluded. We extracted data on study characteristics and length of follow-up, patient-level data on diagnosis, age, sex, race, and admission history, and outcomes of interest (readmission to hospital, inpatient bed-days, community service use, and treatment adherence) for meta-analysis, for which we extracted summary estimates. We used a random-effects model to compare disparate outcome measures and convert effect size statistics into standardised mean differences. This systematic review is registered with PROSPERO, number CRD42018086232.FindingsOf 1931 studies identified, 41 (2%) met inclusion criteria and had sufficient data for analysis. Before and after CCT comparisons showed significant large effects on readmission to hospital (standardised mean difference 0·80, 95% CI 0·53–1·08; I2=94·74), use of community services (0·83, 0·46–1·21; I2=87·26), and treatment adherence (2·12, 1·69–2·55; I2=0), and a medium effect on inpatient bed-days (0·66, 0·46–0·85; I2=94·12). Contemporaneous controlled comparison studies (randomised and non-randomised) showed no significant effect on readmission, inpatient bed-days, or treatment adherence, but a moderate effect on use of community services (0·38, 0·19–0·58; I2=96·92). A high degree of variability in study quality was found, with observational study ratings ranging from three to nine. Bias most frequently centred on poor comparability between CCT and control participants.InterpretationWe found no consistent evidence that CCT reduces readmission or length of inpatient stay, although it might have some benefit in enforcing use of outpatient treatment or increasing service provision, or both. Future research should focus on why some people do not engage with treatment offered and on enhancing quality of the community care available. Shortcomings of this study include high levels of variability between studies and variation in study quality.FundingNational Institute for Health Research.