Little is known about the prevalence of psychiatric illness in elderly people with mental handicaps. Behaviour disorders occur less often in the elderly compared to younger groups, but which other illnesses they are vulnerable to or protected from is unknown. Associated characteristics are not clear, not is the interrelationship with social factors. Issues of treatment have not been addressed. Postmortem studies in Down's syndrome suggest almost 100% of individuals over 40 to have neuropathological changes of Alzheimer's disease, but this is probably an overestimation of the true prevalence. Attempts to link neuropathological findings with clinical findings have been disappointing; clinical dementia is elusive. When a decline in skills has been elicited, studies have often overlooked the differential diagnosis, making a presumptive diagnosis of Alzheimer's disease. To date there has been no comprehensive prospective study with neuropathological correlates in the field of Down's syndrome and dementia. People with mental handicaps are living longer, and hence the elderly are emerging as a new group in our society. The impact that this will have on services cannot be predicted and planning of provisions cannot be made until there is more knowledge about this group of individuals.