A report published in Wednesday's Journal of the American Medical Association offers circumstantial evidence that hormone replacement therapy causes breast cancer. During the study, researchers followed 190,458 women age 30 and over, determining that the rate of two types of invasive breast cancer increased 65 percent from 1987 to 1999. The two types of disorders, lobular and lobular-ductal mixed, are strongly linked to hormone use. Researchers do not know how many of the study's participants were using hormone replacement medications during the inquiry period. According to the report, however, the study occurred at a time when an increasing number of women began taking estrogen and progestin hormones.

In July 2002, The National Institute of Health (NIH) stopped a 16,608-woman study on the benefits of hormone replacement therapy three years early, concluding that hormones such as estrogen and progestin place women who still have their uterus at risk of developing invasive breast cancer.