In 1948, when Evelyn and Aurele DesRosiers joined the study at the urging of a neighbor, they knew only that they were being promised an extraordinary level of medical care. And indeed, they soon found themselves undergoing what struck them as astonishingly thorough examinations, with blood tests, X-rays, electrocardiographs and even electrokymographs, a technology that showed the image of a beating heart but was destined for the medical scrap heap.

For Evelyn, now 94 years old, it was the first of 29 biennial exams and the beginning of a three-generation involvement with Framingham. Her family’s story reads like a brief history of American health care during the second half of the twentieth century. Aurele, a lifelong smoker, had his first heart attack in 1963, at age 55, and though he then gave up cigarettes and took up walking for exercise, his doctors didn’t know to advise him to lay off the bacon and eggs, and he died in 1976 after several more heart attacks. A son, Ronald DesRosiers, now 71, joined Framingham in 1971 and was also a smoker—startlingly, on the recommendation of a doctor who suggested he start after a bout of asthma. Eventually, though, he became more health-conscious, working with his physicians to reduce his cholesterol and control his hypertension. His sister, Anne Marie Bouchard, and her husband, Armand, who return regularly from Maine for Framingham exams, have embraced the study’s findings. They’re longtime walkers, sailors and cross-country skiers, and they make sure to eat healthy foods. Their daughter, Sharon Wade, 41, who jumped at the chance to become a third-generation Framingham participant, also watches her diet, exercises several times a week and appreciates being on the front lines of preventive medicine. “If there’s a new test to assess risk of stroke or heart attack, we’ll get it at Framingham,” Wade says.

That, of course, is the Framingham bargain: Those who participate will get the best medicine has to offer and in turn will provide data that helps expand medicine’s frontiers. The decision to recruit the children and grandchildren of the study’s original participants—adding 5,124 second-generation subjects in 1971 and 4,095 from the third generation in 2002—only extended that covenant of mutual interest in the hope of breaking still more ground. These days, that inevitably involves going beyond risk factors to seek to understand the genetic root causes of disease.

That would have been an unimaginable concept when the study was launched. “DNA hadn’t even been discovered,” Kannel says. “All we knew back then was that certain diseases might be inherited.” Starting in the mid-1990s, Framingham researchers began collecting DNA samples from participants, and in 2003 the Human Genome Project, run by the U.S. Department of Energy and the National Institutes of Health, announced it had identified the 20,000 genes in human DNA and determined the correct order of the four chemical units found in DNA’s base pairs—abbreviated as A, T, C and G—that constitute each of the 23 chromosome pairs of a human cell. Those achievements meant that more than a half century of tracking families and disease could now be put to a new, possibly revolutionary use.