Psychiatric acute units use different levels of segregation to satisfy needs for containment and decrease in sensory input for behaviourally disturbed patients. Controlled studies evaluating the effects of the procedure are lacking. The aim of the present study was to compare effects in acutely admitted patients with the use of a psychiatric intensive care unit (PICU) and not in a psychiatric acute department. In a naturalistic study, one group of consecutively referred patients had access only to the PICU, the other group to the whole acute unit. Data were obtained for 56 and 62 patients using several scales. There were significant differences in reduction of behaviour associated with imminent, threatening incidents (Broset Violence Checklist), and actual number of such incidents (Staff Observation Aggression Scale-Revised) in favour of the group that was treated in a PICU. The principles of patient segregation in PICUs have favourable effects on behaviours associated with and the actual numbers of violent and threatening incidents.