Despite the demands it places on individuals, families and the community, intellectual disability (ID) is a neglected area of public health. Accurate estimates of prevalence are sparse and range from 0.5 to 3.0%. The cause of the condition is unknown in at least 50% of cases. This paper describes the Intellectual Disability Exploring Answers (IDEA) database set up in Western Australia to provide an infrastructure for research and to facilitate the planning of service provision for people with ID. Since 1953 a database for ID has been maintained in Western Australia, a state with a population of 1.95 million in an area of 2.52 million km2. The current IDEA database aims to obtain ongoing population-based ascertainment of ID from providers of clinical and educational services, with the potential for linkage to a network of other state databases. The average prevalence of ID for children born in Western Australia over the years 1983-1996 was 15.2 per 1000 live births, with 50% ascertained only through the education system. During this time period 60% of cases were male. Of children with an ID born in Western Australia in 1980-1999 and surviving to 1 year, 30.1% had a birth defect, and the prevalence ratio of birth defects in this group compared to the population with no birth defects was 6.5 (CI 6.3-6.8).