WASHINGTON (AP) _ Privately financed researchers will begin this fall studying whether taking estrogen could protect middle-aged women against heart disease.

The hormone has had a series of headline-grabbing failures in older women.

Doctors long thought that hormone therapy would keep women generally healthier after menopause by reducing heart attacks and keeping the brain sharp. Starting in 2002, however, major studies from the National Institutes of Health concluded that hormone therapy not only does not prevent heart disease, but long-term use raises the risk of strokes and other problems.

Millions of women have abandoned the therapy.

The average participant in those studies was in her 60s; menopause generally begins in the 40s or 50s. So critics wonder if the results pertain to younger women as well, an important question as estrogen remains crucial to treating menopause symptoms such as hot flashes.

The new study will track 720 women ages 40 to 55 over five years, to see if hormone therapy could slow thickening arteries and other early signs of developing heart disease.

That's far too few women for a sure answer, cautions NIH's Dr. Jacques Rossouw. After all, the NIH studies involved more than 26,000 women.

But if the results should prove positive, it might spur larger studies to settle the age question.

No previous studies have proved hormones increase heart risk when women take them at menopause, and one NIH study suggested 50-somethings might have a slightly lower risk, said Dr. JoAnn Manson of Brigham and Women's Hospital. She helped lead the NIH research and the new private study, too.

The new study will compare estrogen pills and patches with dummy pills. Enrollment is expected to begin in September.

The study will be headed by Dr. Mitchell Harman of the nonprofit Kronos Longevity Research Institute in Phoenix. The funding, more than $12 million, comes from a foundation formed by billionaire John Sperling, founder of the University of Phoenix, who also known for financing projects to research pet cloning.