The UK Biobank, the most comprehensive health study in the UK, is opening its doors to researchers.
It has collated about 20TB (terabytes) of securely stored data, the equivalent of 30,000 CDs-worth, on 500,000 people.
The aim of the biobank is to improve the prevention, diagnosis and treatment of a wide range of conditions such as heart disease, cancer and diabetes.
England's chief medical officer, Dame Sally Davies, said the UK Biobank would be a "globally unique resource".
The biobank began recruiting participants three years ago, and was open to people aged 40 to 69.
Each answered questions on their health, lifestyle, diet, memory, work and family history.
They also had a range of measurements taken, including blood pressure, pulse rate, height, weight, body fat and lung function, and provided blood, saliva and urine samples.
'Very exciting day'
About 26,000 people with diabetes, 50,000 with joint disorders, 41,000 teetotallers and 11,000 heart attack patients are taking part.
Participants' health will be followed over many years.
It is about to undertake repeat measures of 20,000 participants from the Manchester area, and later this year it will ask participants to wear an activity monitor for one week.
It has also been overseeing a diet questionnaire, which has been filled in by 400,000 participants.
The hope is that the UK Biobank will allow scientists to investigate why some people develop particular diseases in middle age while others do not, with the hope of developing new treatments and prevention strategies.
It will be open to researchers from the UK and abroad, who will be able to use the - anonymised - data in their work.
Professor Sir Rory Collins, principal investigator at the biobank, said: "This is without doubt a very exciting day for medical research, not just in the UK but around the world.
"We are grateful to participants for their trust and support so far. But they have not joined the project to see it remain idle; we all want to see the resource used extensively to bring about benefits to health and wellbeing."
Prof Dame Sally Davies, chief medical officer and chief scientific adviser at the Department of Health, said: "UK Biobank is a globally unique resource which places the UK at the forefront of the quest to understand why some people develop life-threatening diseases or debilitating conditions.
"It has huge potential for future generations and will help us understand how our children and our children's children can live longer, healthier lives."
The biobank is funded by the Wellcome Trust, Medical Research Council, Department of Health, Scottish Government, Welsh Government and the British Heart Foundation.