A study at the National Cancer Institute (NCI) has shows that women who used estrogen replacement therapy after menopause were at increased risk for ovarian cancer. The scientists followed 44,241 women for approximately 20 years. Compared to postmenopausal women not using hormone replacement therapy, users of estrogen-only therapy had a 60 percent greater risk of developing ovarian cancer. The risk increased with length of estrogen use.

The women, who were followed from 1979 to 1998, were former participants in the Breast Cancer Detection Demonstration Project, a mammography screening program conducted between 1973 and 1980.

Estrogen is a natural hormone produced primarily by the ovaries. After menopause, the ovaries produce lower levels of the hormones estrogen and progesterone. By the time natural menopause is complete - usually between ages 45 and 55 - hormone output decreases significantly. As early as the 1940s, women began using estrogens in high doses to counteract some of the short-term discomforts of menopause (hot flashes, vaginal drying and thinning, and urinary tract incontinence and infections).

However, after it became clear in the 1970s that women who took estrogen alone had a six to eight times higher risk of developing endometrial cancer (cancer of the lining of the uterus), doctors began prescribing progestin along with much lower doses of estrogen. Progestin is a synthetic form of the natural hormone progesterone. The addition of progestin to estrogen therapy reduces the increased risk of endometrial cancer associated with using estrogen alone. As a result, it has become increasingly common to prescribe estrogen-progestin therapy for women who have not had a hysterectomy.

Previous studies looking at the effect of postmenopausal hormones on ovarian cancer risk have been inconsistent. Some reported increased risk with estrogen use while others reported either no effect or a protective one. Most of these earlier studies were relatively small and limited by incomplete information about ovarian cancer risk factors.

An estimated 40 million U.S. women will experience menopause during the next 20 years, and women today are living approximately one-third of their life after menopause.

Anywhere from 20 percent to 45 percent of U.S. women take some form of hormone therapy between the ages of 50 and 75. According to industry estimates, about 8 million U.S. women use estrogen alone and about 6 million U.S. women use estrogen-progestin therapy. About 20 percent of hormone users continue for more than five years.