Long-term estrogen therapy may be related to a higher risk of breast cancer among postmenopausal women who have had a hysterectomy, according to an article in the journal Archives of Internal Medicine.

Previous studies have linked the use of hormone therapy to breast cancer among postmenopausal women, but have primarily focused on the hormone combination of estrogen plus progestin, according to background information in the article. Recently released results from the Women's Health Initiative (WHI), a large clinical trial of hormone therapy, found no significant link between estrogen therapy and breast cancer in women who took the hormone for seven years.

Wendy Y. Chen, M.D., M.P.H., Brigham and Women's Hospital and Dana Farber Cancer Institute, Boston, and colleagues evaluated women who were part of the Nurses' Health Study, a group of female nurses that have been followed since 1976. In 1980, 11,508 women from the study were postmenopausal and had had a hysterectomy. Every two years the researchers enrolled all the additional women who became postmenopausal and had a hysterectomy, so 28,835 women were included by the end of the study in 2002.

Women were asked by questionnaire every two years if they used hormones and whether they had developed breast cancer. For women who developed breast cancer, the researchers obtained permission to review the women's medical records, which they used to record the hormone receptor information. Tumors were classified as positive or negative for estrogen receptor or progesterone receptor based on how they responded to specific hormonal therapies.