There is controversy as to whether compulsory community treatment for people with severe mental illnesses reduces health service use, or improves clinical outcome and social functioning. Given the widespread use of such powers it is important to assess the effects of this type of legislation.To examine the clinical and cost effectiveness of compulsory community treatment for people with severe mental illness.We undertook searches of the Cochrane Schizophrenia Group Register 2003, 2008, and Science Citation Index. We obtained all references of identified studies and contacted authors of each included study.All relevant randomised controlled clinical trials of compulsory community treatment compared with standard care for people with severe mental illness.We reliably selected and quality assessed studies and extracted data. For binary outcomes, we calculated a fixed effects risk ratio (RR), its 95% confidence interval (CI) and, where possible, the weighted number needed to treat/harm statistic (NNT/H).We identified two randomised clinical trials (total n = 416) of court-ordered 'Outpatient Commitment' (OPC) from the USA. We found little evidence that compulsory community treatment was effective in any of the main outcome indices: health service use (2 RCTs, n = 416, RR for readmission to hospital by 11-12 months 0.98 CI 0.79 to 1.2); social functioning (2 RCTs, n = 416, RR for arrested at least once by 11-12 months 0.97 CI 0.62 to 1.52); mental state; quality of life (2 RCTs, n = 416, RR for homelessness 0.67 CI 0.39 to 1.15) or satisfaction with care (2 RCTs, n = 416, RR for perceived coercion 1.36 CI 0.97 to 1.89). However, risk of victimisation may decrease with OPC (1 RCT, n = 264, RR 0.5 CI 0.31 to 0.8). In terms of numbers needed to treat (NNT), it would take 85 OPC orders to prevent one readmission, 27 to prevent one episode of homelessness and 238 to prevent one arrest. The NNT for the reduction of victimisation was lower at six (CI 6 to 6.5). A new search for trials in 2008 did not find any new trials that were relevant to this review.Compulsory community treatment results in no significant difference in service use, social functioning or quality of life compared with standard care. People receiving compulsory community treatment 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. Evaluation of a wide range of outcomes should be considered when this type of legislation is introduced.