Researchers at the University of California, Los Angeles, School of Medicine came to that conclusion after studying mammograms of 571 women enrolled in the Postmenopausal Estrogen/Progestin Interventions Trial, a randomized clinical trial investigating the effects of various hormone replacement treatments. Some of the women took only estrogen therapy, some took a combination of estrogen and progestin and some took only a placebo.

As reported in the Jan. 1 issue of the Journal of the National Cancer Institute, after analyzing the data and mammograms, researchers found the estrogen/progestin combination therapy, regardless of how the progestin was given in the treatment, was associated with a 3- to 5-percent increase in breast density. The use of estrogen alone was not linked to any increases in breast density.

"The important aspect of this research is that we see changes in mammograms that reflect what we know about combined estrogen and progestin use and breast cancer risk," researcher and study co-author Dr. Giske Ursin, of the Department of Preventive Medicine at Norris Comprehensive Cancer Center at the University of Southern California, told United Press International.

"The challenge now is to determine whether the women that have the largest mammographic density changes are the ones who go on to develop breast cancer," Ursin said. "Although we know mammographic density in itself is a risk factor for breast cancer, we do not know if a change in density translates to a change in risk."

Ursin added: "Because we do not know if a change in density translates to a change in risk of breast cancer, I do not think we can make recommendations for women on whether they should stop their hormone therapy based on our paper."

Dr. Victoria Seewaldt, director of breast cancer prevention at Duke University in Durham, N.C. called the research a fascinating study.

"Biologically, it really threw me for a loop," Seewaldt told UPI, "because we really don't understand how estrogen and progestin (behave) in the breast." Also, scientists do not understand how progestin alone acts on breast tissue, she explained. The finding, "argues for a lot more research because we don't really understand the biology of a normal breast," she said.

Many women have been confused about whether to take hormone-replacement therapy following reports last summer that found several serious side effects associated with HRT.

"The major reason I can see taking HRT is to combat menopausal symptoms and this is something I think is under-appreciated by the medical system how severe these symptoms can be," Seewaldt said. "Women have to really understand what we don't know about these drugs before embarking on HRT."

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(Reported by Katrina Woznicki, UPI Science News, in Washington.)