It is controversial whether compulsory community treatment (CCT) for people with severe mental illness (SMI) reduces health service use, or improves clinical outcome and social functioning. To examine the effectiveness of compulsory community treatment (CCT) for people with severe mental illness (SMI). We searched the Cochrane Schizophrenia Group's Study-Based Register of Trials (2003, 2008, 2012, 8 November 2013, 3 June 2016). We obtained all references of identified studies and contacted authors where necessary. All relevant randomised controlled clinical trials (RCTs) of CCT compared with standard care for people with SMI (mainly schizophrenia and schizophrenia-like disorders, bipolar disorder, or depression with psychotic features). Standard care could be voluntary treatment in the community or another pre-existing form of CCT such as supervised discharge. Authors independently selected studies, assessed their quality and extracted data. We used Cochrane's tool for assessing risk of bias. For binary outcomes, we calculated a fixed-effect risk ratio (RR), its 95% confidence interval (95% CI) and, where possible, the number needed to treat for an additional beneficial outcome (NNTB). For continuous outcomes, we calculated a fixed-effect mean difference (MD) and its 95% CI. We used the GRADE approach to create 'Summary of findings' tables for key outcomes and assessed the risk of bias of these findings. The review included three studies (n = 749). Two were based in the USA and one in England. The English study had the least bias, meeting three out of the seven criteria of Cochrane's tool for assessing risk of bias. The two other studies met only one criterion, the majority being rated unclear.Two trials from the USA (n = 416) compared court-ordered 'outpatient commitment' (OPC) with entirely voluntary community treatment. There were no significant differences between OPC and voluntary treatment by 11 to 12 months in any of the main health service or participant level outcome indices: service use - readmission to hospital (2 RCTs, n= 416, RR 0.98, 95% CI 0.79 to 1.21, low-quality evidence); service use - compliance with medication (2 RCTs, n = 416, RR 0.99, 95% CI 0.83 to 1.19, low-quality evidence); social functioning - arrested at least once (2 RCTs, n = 416, RR 0.97, 95% CI 0.62 to 1.52, low-quality evidence); social functioning - homelessness (2 RCTs, n = 416, RR 0.67, 95% CI 0.39 to 1.15, low-quality evidence); or satisfaction with care - perceived coercion (2 RCTs, n = 416, RR 1.36, 95% CI 0.97 to 1.89, low-quality evidence). However, one trial found the risk of victimisation decreased with OPC (1 RCT, n = 264, RR 0.50, 95% CI 0.31 to 0.80, low-quality evidence).The other RCT compared community treatment orders (CTOs) with less intensive and briefer supervised discharge (Section 17) in England. The study found no difference between the two groups for either the main health service outcomes including readmission to hospital by 12 months (1 RCT, n = 333, RR 0.99, 95% CI 0.74 to 1.32, moderate-quality evidence), or any of the participant level outcomes. The lack of any difference between the two groups persisted at 36 months' follow-up.Combining the results of all three trials did not alter these results. For instance, participants on any form of CCT were no less likely to be readmitted than participants in the control groups whether on entirely voluntary treatment or subject to intermittent supervised discharge (3 RCTs, n = 749, RR for readmission to hospital by 12 months 0.98, 95% CI 0.82 to 1.16 moderate-quality evidence). In terms of NNTB, it would take 142 orders to prevent one readmission. There was no clear difference between groups for perceived coercion by 12 months (3 RCTs, n = 645, RR 1.30, 95% CI 0.98 to 1.71, moderate-quality evidence).There were no data for adverse effects. These review data show CCT results in no clear difference in service use, social functioning or quality of life compared with voluntary care or brief supervised discharge. People receiving CCT were, however, less likely to be victims of violent or non-violent crime. It is unclear whether this benefit is due to the intensity of treatment or its compulsory nature. Short periods of conditional leave may be as effective (or non-effective) as formal compulsory treatment in the community. Evaluation of a wide range of outcomes should be considered when this legislation is introduced. However, conclusions are based on three relatively small trials, with high or unclear risk of blinding bias, and low- to moderate-quality evidence. In addition, clinical trials may not fully reflect the potential benefits of this complex intervention.