BackgroundInformation on the numbers of adultpersons (aged20 years and over) with intellectualdisability (ID) is rarely collated at a national level.This is an impediment to service planning especiallyfor a changing population.MethodsA database of all persons in receipt of IDservices has been operating in the Republic of Irelandsince1995. In Northern Ireland, regional databasescan be used to provide similar information.ResultsA total of25, 134persons were known toservices in2002; an overall prevalence for the islandof6.34 per1,000. However this rate varied for differentage groupings and across the two parts of theisland. General population characteristics, as well asservice factors, appear to account for this. Significantlymore people lived with family carers in NorthernIreland. By2021, it was estimated that thepopulation would increase by over20 % with aroundone-third of persons aged over50 years.ConclusionsThese data illustrate the variations thatexist in the numbers of adult persons with ID knownto services across and within regions of a country.Hence caution must be exercised in extrapolatingprevalence rates derived in one area to another. Theavailability of comparative national data highlightsissues around the equitable funding and delivery ofservices.