n 2003, Britain plans to undertake the world's most ambitious study of the origins of disease.

Looking forward to the day when people will know their genetic makeups and request a precise picture of their risks of developing various diseases, the study organizers plan to assemble a database of medical information about 500,000 Britons, including their DNA.

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The goal, over the next 10 to 20 years, is to sort out the way that genes and the environment combine to cause common diseases.

At least six countries are preparing similar databases, in essence genetic censuses, or have already established them. Iceland, the pioneer, has collected medical data and DNA samples from 80,000 related people to hunt genes that touch off disease. It will also develop a national database from patient records in its health care system.

DNA-based diagnostics and drugs could result from the effort.

Britain's will be the largest of the databases proposed by governments or their private partners. It has the same goals as Iceland's but with a critical difference: for the first time, it will try to quantify the roles of genes and environmental influences like smoking, alcohol, viruses, pollution, exercise and diet in unrelated people for all common diseases. Studying a huge selection of diverse people could make its discoveries applicable worldwide. Iceland's population, by contrast, is extraordinarily homogeneous.

If the $120 million project, called U.K. Biobank, goes forward, and enough people volunteer for pilot studies, 1.2 million healthy Britons from 45 to 69 will give blood samples to the Biobank. From their blood, DNA will be purified and frozen. Ninety percent of the donors will be white. The rest will roughly reflect Britain's demographics.

From these, 500,000 will be chosen for the project by 2008.

When they sign up, volunteers will get brief health examinations and will answer 10-page questionnaires about their socioeconomic and psychological status, reproductive history, exercise, cellphone use and beverage preferences. They will note their diets for a week.

For 10 years, they will be followed through their national health care records, which will be copied into the Biobank. The data will be anonymous, but not completely, to allow for updates by doctors or new questionnaires. By 2014, 40,175 are expected to fall ill with diabetes, heart disease, stroke or cancer. Another 6,200 are expected to have Parkinson's, dementia, rheumatoid arthritis or hip fractures.

The DNA of these people will be read and compared, and any normal gene variants, the one-nucleotide differences in DNA that make one person's biology different from another's, will be tagged for study.

"Then you will be able to see patterns: X number have this sort of genetic makeup and this kind of lifestyle, and Y has that, and you can start analyzing, if you like, the nature-nurture, environment-genes secret," said Sir George Radda, the molecular cardiologist who heads the Medical Research Council, a sponsor of the Biobank.

So far, opposition has been muted and polite. But a significant minority of British doctors oppose the project as unnecessary and too costly.

American geneticists are also split on the value of huge medical and DNA databases. Some argue that existing ones like the Framingham Heart Study, which is gathering DNA from descendants of its original subjects, are enough. Others question their design.

Dr. David Altshuler, a geneticist at the Whitehead/M.I.T. Center for Genome Research and Harvard Medical School, said, "I am not sure that a one-size-fits-all gene bank is what we want, but it is absolutely necessary to do prospective population studies if we are going to give any valuable information to the average patient who walks into their doctor and says, `Does this genetic discovery I read about in the paper apply to me?' or `What does it mean if I have a certain gene variant?' "

Health officials in the United States are beginning to discuss a large database for research, said Dr. Lisa Brooks, a spokeswoman for the National Genome Research Institute at the National Institutes of Health. Smaller databases, gathering DNA from volunteers, are being started by the Mayo Clinic, among others.