In 1948 life was good, or so it seemed. The postwar economic boom had transformed America into a land of plenty, with cars, televisions, cigarettes and second helpings of meat and eggs for just about anyone who wanted them. But prosperity came at a price. People were dropping dead of heart attacks, and physicians in the 1940s had limited understanding of the cardiovascular system, let alone of treatments for heart disease. A quarter of heart attacks were recorded on death certificates as “acute indigestion” or “cause unknown,” according to Daniel Levy, FHS’s current director and author of A Change of Heart: Unraveling the Mysteries of Cardiovascular Disease.

Yet, if general practitioners of the day weren’t willing (or able) to recognize that at least half their patients were dying from heart attacks and stroke, the U.S. Public Health Service was sounding the alarm. In 1948 it sponsored a 20-year epidemiological study that would follow a group of healthy people in one community and observe changes in their health, including the development of heart disease. The government agency provided initial funding of $500,000.

Such a study of a chronic disease had never been attempted in the United States, though a few years earlier, cardiologist Paul Dudley White at the Massachusetts General Hospital had begun following men who’d already had heart attacks. White had hoped to find the causes by backtracking through medical histories and information about the men’s lifestyles. But spotty memories confounded him. “He concluded that the study would have to be done the other way around—to start with healthy people and wait for them to develop heart disease,” Levy says.

So Framingham would look forward, not back, recruiting 5,209 healthy subjects—about half the town’s population between ages 30 and 60—and systematically recording data on their diet, physical activity, smoking, family history and medications. Each participant also underwent an extensive physical examination every two years. Women were included in the study as a kind of control group. Because they were mistakenly thought to have some kind of innate protection against heart attacks, women were expected to teach useful lessons that could be applied to men.

When the town of Framingham’s physicians learned that their patients were to be recruited for a federally funded project, they complained that they didn’t want the government peering into their practices, and they worried they would lose their patients to the study’s doctors. So the first director, Thomas Royal “Roy” Dawber, promised that Framingham would never treat participants but rather would notify physicians of any findings that required their attention. And the town’s physicians were invited to be the first to receive a complete FHS exam, so they could answer patients’ questions about the study.

If Dawber was successful in convincing community physicians of the importance of a heart study, he and the other Framingham researchers had a much bigger challenge in getting them to believe the data. Then-prevailing views of heart disease held that it was either a natural part of aging or a specific biological phenomenon with a single cause—in the way that tuberculosis, for example, is directly tied to the tubercle bacillus. But as study data trickled in, Framingham researchers began to hypothesize that chronic diseases such as stroke and diabetes result instead from several interrelated causes—“factors of risk,” as Dawber and Kannel termed them, coining a variation of the phrase now at the heart of any conversation about disease and how to prevent it.

“Framingham showed you can take measurements done in any physician’s office, such as tests of cholesterol, and predict the probability that someone will have a heart attack or a stroke within 10 years,” says Philip A. Wolf, who joined FHS in 1967 as a stroke researcher and is now its principal investigator. “But the study also demonstrated that if people make particular changes in their lifestyles, they can substantially reduce their risk of disease.” It was a revolutionary idea—and the dawn of preventive medicine.