Around 100,000 Britons are to be invited to undergo detailed imaging of their brain, heart and other vital organs as part of the world's biggest scanning project.
All the volunteers will already be part of UK Biobank, and have supplied DNA and given detailed health information.
The scans will be compared and cross-referenced with other data.
The aim is to try to improve the diagnosis and treatment of a huge range of diseases.
Currently 500,000 adults aged 40-69 are enrolled in UK Biobank which is supported by the Wellcome Trust, Medical Research Council, British Heart Foundation and governments across the UK.
When they enrolled they gave blood and urine samples, underwent fitness and cognitive tests, and answered detailed lifestyle questions.
Now one in five participants will be asked to give up half a day and undergo a series of scans including MRI, X-ray and ultrasound.
The scans are non-invasive and the amount of radiation from the low-level X-ray is similar to that emitted by an airport security scanner.
Prof Sir Rory Collins, Chief Executive UK Biobank said: "We are trying to understand why one person gets a disease and another does not. The scanning information, when analysed alongside all the existing health data, will give researchers a unique opportunity to study the causes of ill-health."
The scanning project is not a healthcheck, and no feedback will be given to participants.
They will not get to see their scans, which, like the other health data, will be anonymised.
However, if a potentially serious abnormality is discovered then researchers will write to the volunteer and contact their GP.
UK Biobank expects to find potentially serious problems in 10-15% of volunteers.
Many of these will turn out to be benign, but others will require treatment.
Prof Cathie Sudlow, the chief scientist at UK Biobank said: "Early diagnosis can be very beneficial as many cancers respond best to early treatment."
However, she said there were also potential downsides to getting such information.
"Some cases we will come across findings where it remains uncertain or where the problem is not amenable to treatment.
"So someone has to live with the knowledge that they have something that nothing can be done about, which can create worry and have a negative impact on travel and life insurance."
UK Biobank accepts that this may deter some people from taking part, and says it is already hugely grateful to volunteers.
Prof Collins said: "UK Biobank is a remarkable example of altruism. Participants have got involved not for themselves, but to improve the health of future generations".
I was the first volunteer to undergo the scans, at UK Biobank headquarters in Stockport.
The whole process takes about four hours and includes a repeat of some of the baseline measurements done on enrolment, such as height, weight and blood pressure.
None of the tests is unpleasant. For those who are wary about MRI body scans, the machines used by UK Biobank have a wider aperture than standard scanners, reducing any sense of claustrophobia.
It is hoped that the brain imaging may yield clues as to the causes of dementia and help improve early diagnosis.
UK Biobank will also be inviting around a third of a million participants to repeat online cognitive tests they took on enrolment.
The database is open to researchers worldwide.
In the past two years studies have begun into a whole range of conditions, including cancer, heart disease, depression, arthritis, hearing loss, stroke, diabetes, lung function and allergies.
Scientists at UK Biobank have begun to analyse the DNA of all 500,000 participants.
These will be identifying 850,000 biomarkers, many of which are associated with diseases.
Combining and cross-referencing the genetic information with the structural images of the body will give scientists a huge amount of comparative data which will be the biggest resource of its kind in the world.
The data will become increasingly powerful in years to come as researchers feedback their results and participants develop diseases associated with ageing.
The scanning project will initially be limited to participants who live in the Manchester area who can visit the facility in Stockport.
After an initial pilot stage, further scanning facilities will open around the UK, with the whole project due for completion in around five years.
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