Doctors who want to identify women at risk of heart disease should test her fitness, say researchers.
Keeping fit cuts health problems
Scientists from Johns Hopkins University in Baltimore found treadmill tests could help to identify vulnerable women.
They proved to be more reliable than more sophisticated electrical analysis of the heart.
Nearly 3,000 women took part in the study published in the Journal of the American Medical Association.
The researchers focused on two measurements of fitness: how long a woman could exercise on a treadmill, and how long it took her heart to return to normal once she had stopped.
Women who performed below average on both measures were 3.5 times more likely to die of heart disease than women who were above average.
However, among women who had been thought to be a low risk, those who scored below average were nearly 13 times more likely to die of heart disease than those who performed better on the tests.
The worse the performance in the exercise tests, the greater the risk proved to be.
In contrast, electrical measurement of decreased blood flow - used to diagnose hidden heart disease in men - did not accurately identify women with hidden heart disease.
Researcher Dr Roger Blumenthal said: "There is great public health interest in cost-effective and readily available tests that can predict cardiovascular risk in asymptomatic women, since nearly two-thirds of women who die suddenly have no previous symptoms."
The researchers stress that their study provides further evidence that regular exercise is an effective way to reduce the risk of heart disease.
Professor Sir Charles George, medical director of the British Heart Foundation, said the study had important messages for women.
"Although we have known for some time that an inactive life increases the risk of coronary heart disease, we had not realised the full extent of this for women.
"Regular exercise reduces the risk of coronary heart disease and one way of identifying how fit you are is by measuring the time it takes for your heart rate to return to normal following exertion.
"This study shows that by recording this measure we can help to detect women at risk of CHD who can then take action to safeguard their health."