A parent's history of premature heart disease, all by itself, is as potent a risk factor for a heart attack or stroke as high cholesterol, high blood pressure or smoking, a major new study shows.

The results come from the 50-year-old Framingham Heart Study. It reflects the first attempt ever to confirm what other studies suggest — that genetics profoundly influence heart disease risk — by following parents and children for decades.

The study found that a middle-aged person is twice as likely to have life-threatening heart disease if his or her parents have been diagnosed early in life, a father by 55, a mother by 65. Having two parents with premature heart disease raises a person's risk even more.

Doctors say the increased risk is independent of other factors such as smoking, high cholesterol, high blood pressure or diabetes. The higher risk shows up most in people between the ages of 30 and 59 when advancing age begins to exert a greater influence.

"This study provides the most accurate available evaluation of risk in men and women," says lead author Christopher O'Donnell, who is associate director of the Framingham study.

Previous studies have relied on children's knowledge of their parents' medical histories, a notoriously unreliable measure. The Framingham study is unique because it began in 1948, enabling researchers to follow parents and their children for decades.

This study, in Wednesday's Journal of the American Medical Association, tracked 1,128 men and 1,174 women.

When the study began the participants were in their mid-40s and were free of heart disease. By the study's end in 2001, 164 men and 79 women had had heart attacks, strokes, angina and other serious heart problems.

Those between the ages of 30 and 59 had a three-fold increased risk of heart disease. People 60 and over had a two-fold increased risk.

James Cleeman, coordinator of the National Cholesterol Education Program at the National Heart, Lung, and Blood Institute, which sponsored the study, said the results underline the importance of factoring family history into a person's 10-year risk of having a heart attack.