Background. For many years, it has been known that dementia can occur in people with learning disabilities, but there have been few research studies. Studies that do quote rates for dementia show these to be high, but this important fact has received remarkably little attention.

Method. Comprehensive psychiatric and medical assessments were undertaken on the whole population (ascertained as far as is possible) of people with learning disabilities aged 65 years and over living in Leicestershire, UK (N=134), in order to ascertain rates of DCR defined dementia, and associated factors.

Results. Dementia was diagnosed in 21·6%, against an expected prevalence of 5·7%, for a group with this age structure. The rate of dementia increased in successive age cohorts: 15·6% aged 65–74 years; 23·5% aged 65–84 years; and 70·0% aged 85–94 years. People with dementia tended to be older, female, with more poorly controlled epilepsy, a larger number of additional physical disorders, less likely to be smokers and had lower adaptive behaviour scores than did the elderly people without dementia. They were more likely to live in health service accommodation.

Conclusions. Dementia occurs at a much higher rate among elderly people with learning disabilities than it does among the general population; this is independent of the association between dementia and Down's syndrome. Whether this relates aetiologically to genetics, lack of brain ‘reserve’ or history of brain damage is yet to be determined.