The sixth national census of Irish psychiatric units and hospitals was carried out on 31 March 2006. All patients resident in psychiatric units and hospitals operating under the provisions of the Mental Treatment Act 1945 were enumerated in this census. The rates presented in this document have been based on the 2002 Census of Population. Shortly before finalising our report the preliminary results of the 2006 Population Census became available. These were limited to numbers and gender with age not becoming available until 2007. In addition to presenting data by age and other variables derived from the 2002 Census we have also rated our numbers to the 2006 all-aged population where appropriate. As the population in Ireland increased by 8.1 per cent in the four year period 2002- 2006 rates reported here will not reflect this increase.