Inpatient treatment is an expensive way of caring for people with acute psychiatric disorders. It has been proposed that many of those currently treated as inpatients could be cared for in acute psychiatric day hospitals. To assess the effects of day hospital versus inpatient care for people with acute psychiatric disorders. We searched the Cochrane Schizophrenia Group Trials Register (June 2010) which is based on regular searches of MEDLINE, EMBASE, CINAHL and PsycINFO. We approached trialists to identify unpublished studies. Randomised controlled trials of day hospital versus inpatient care, for people with acute psychiatric disorders. Studies were ineligible if a majority of participants were under 18 or over 65, or had a primary diagnosis of substance abuse or organic brain disorder. Two review authors independently extracted and cross-checked data. We calculated risk ratios (RR) and 95% confidence intervals (CI) for dichotomous data. We calculated weighted or standardised means for continuous data. Day hospital trials tend to present similar outcomes in slightly different formats, making it difficult to synthesise data. We therefore sought individual patient data so that we could re-analyse outcomes in a common format. Ten trials (involving 2685 people) met the inclusion criteria. We obtained individual patient data for four trials (involving 646 people). We found no difference in the number lost to follow-up by one year between day hospital care and inpatient care (5 RCTs, n = 1694, RR 0.94 CI 0.82 to 1.08). There is moderate evidence that the duration of index admission is longer for patients in day hospital care than inpatient care (4 RCTs, n = 1582, WMD 27.47 CI 3.96 to 50.98). There is very low evidence that the duration of day patient care (adjusted days/month) is longer for patients in day hospital care than inpatient care (3 RCTs, n = 265, WMD 2.34 days/month CI 1.97 to 2.70). There is no difference between day hospital care and inpatient care for the being readmitted to in/day patient care after discharge (5 RCTs, n = 667, RR 0.91 CI 0.72 to 1.15). It is likely that there is no difference between day hospital care and inpatient care for being unemployed at the end of the study (1 RCT, n = 179, RR 0.88 CI 0.66 to 1.19), for quality of life (1 RCT, n = 1117, MD 0.01 CI -0.13 to 0.15) or for treatment satisfaction (1 RCT, n = 1117, MD 0.06 CI -0.18 to 0.30). Caring for people in acute day hospitals is as effective as inpatient care in treating acutely ill psychiatric patients. However, further data are still needed on the cost effectiveness of day hospitals.