"The prevalence is higher than previously thought," says McKinlay, whose research indicated that 10 percent of men ages 40 to 70 had severe ED, while about half had at least mild ED. "We also found a very strong relationship between ED and coronary heart disease that is independent of all other risk factors, such as high cholesterol, hypertension, smoking and diabetes."

Like heart disease, ED is "in some ways a plumbing problem, related to system-wide blockages in blood vessels," he says. "If there's going to be a blockage, it makes sense that it will turn up first in the microvascular penile bed before it turns up in the large blood vessels of the heart."

For this reason, McKinlay calls ED "possibly the best marker we have of coronary heart disease, the biggest killer of aging men." Yet he says "most primary care physicians routinely dismiss this very common problem" as an inevitable part of aging without considering the possibility that it may be a symptom of a life-threatening illness.

The Massachusetts Male Aging Study found a direct correlation between ED and physical activity, with sedentary men at highest risk for impotence and active men at lowest risk. The data suggest that sedentary men may be able to reduce their risk of developing ED by adopting regular physical activity equivalent to taking a brisk two-mile walk each day.

More evidence that exercise may improve men's sexual performance is offered by new research from the Harvard School of Public Health.