Up to one-third of a million adults in the UK are set to take part in a series of memory and reasoning tests to see whether or not certain factors increase the likelihood of developing dementia.
The Medical Research Council is funding this study, which BBC News reports is the largest cognitive function study in the world.
Participants, who are all members of UK Biobank, will be asked to do a number of memory and reasoning tests online.
All those in the trial will have already donated blood and urine samples, carried out an exercise test and given details about their health and what they eat.
In addition, they sat some computer-based puzzles and these will be repeated to see whether or not there is any change in the results.
Epidemiologist at Cardiff University Dr John Gallacher, who helped formulate the tests, said: "Most people will have just minute falls in their test results since they did them last time, but even this might help us predict who will develop dementia in the future."
While Dr Gallacher stressed this was not a dementia-specific test, he said that it was important to know the participants' cognitive function before they developed the condition.
Dr Gallacher remarked: "If we could delay the onset of dementia by five years, that would halve the number of people with the condition".
Alzheimer's Society director of research and development Dr Doug Brown was quoted as saying that studying participants in mid-life could be really helpful in finding clues that help scientists to understand - or even prevent - the condition developing.
This is because the brain changes a number of years before symptoms of the degenerative disease make themselves apparent.
UK Biobank is a major national health resource, which aims to learn more about certain diseases and devise potential cures for them.
Find out about dementia care and support services at Barchester care homes.