Since 1974 a psychiatric hospital security unit, designed to serve the whole catchment area, has cared for mentally ill (mostly psychotic) patients with disturbed behaviour that cannot be managed in open wards. There are a few long-term dangerous patients but most stay only briefly. The admission of women to the unit was not followed by the expected reduction in violence. The unit has facilities for occupational therapy, physical recreation, work, and study, which are particularly important for those who are too dangerous to leave it. The unit's calming influence depends as much on the supportive effect of the high staff ratio as on the use of tranquillisers. This type of unit is not suitable for patients with personality disturbances who "act out" or for mentally abnormal offenders; but it functions well as a crisis centre for the disturbed mentally ill, and there is an increasing demand for its services.