In high-income countries, over the last three decades, the length of hospital stays for people with serious mental illness has reduced drastically although considerable variation remains. In lower-income countries this variation may be greater. Some argue that reduction in hospital stay leads to 'revolving door admissions' and worsening mental health outcomes despite apparent cost savings, whilst others suggest longer stays may be more harmful by institutionalising people to hospital care. To evaluate the effect of short stay/brief admission hospital care with long stay/standard in-patient care in people with serious mental illness. We searched the Cochrane Schizophrenia Group's register of trials, July 2007 and updated this search in May 2012. We included all randomised controlled trials comparing planned short/brief with long/standard hospital stays for people with serious mental illnesses. We extracted data independently. For dichotomous data we calculated risk ratios (RR) and their 95% confidence intervals (CI) on an intention-to-treat basis based using a fixed-effect model. For continuous data, had we identified such data, we planned to calculate fixed-effect mean differences (MD). We assessed risk of bias for included studies and rated quality of evidence using GRADE. We included six relevant trials undertaken between 1969 and 1980. We found no significant difference in death (n = 175, 1 RCT, RR in the longer term 0.42, CI 0.10 to 1.83, very low quality evidence). In the long term, there was no difference in improvement of mental state (n = 61, 1 RCT, RR 3.39, CI 0.76 to 15.02, very low quality evidence). There was no difference in readmission to hospital (n = 651, 4 RCTs, RR by the long term 1.26, CI 1.00 to 1.57, low quality evidence). Data for leaving the study prematurely by the longer term showed no difference (n = 229, 2 RCTs, (RR 0.77, CI 0.34 to 1.77, low quality evidence). There was a significant difference favouring short stay (P = 0.01) in numbers of participants with delayed discharge from hospital exceeding the time planned in study (n = 404, 3 RCTs, RR in the longer term 0.54, CI 0.33 to 0.88, low quality evidence). There was no difference in numbers of participants lost to follow-up (n = 404, 3 RCTs, RR by the longer term 1.07, CI 0.70 to 1.62, low quality evidence). Finally, there was a significant difference favouring short-stay hospitalisation for social functioning, including unemployment, unable to housekeep, or unknown employment status (n = 330, 2 RCTs, RR by longer term 0.61, CI 0.50 to 0.76, very low quality evidence). The effects of hospital care and the length of stay is important for mental health policy. We found limited low and very low quality data which were all over 30 years old. Outcomes from these studies do suggest that a planned short-stay policy does not encourage a 'revolving door' pattern of admission and disjointed care for people with serious mental illness. More large, well-designed and reported trials are justified especially where a short-stay policy is not routine care.