Researchers from the National Cancer Institute in Bethesda, MD have found that combined estrogen-progestin replacement therapy is associated with a greater risk of breast cancer than estrogen alone. Both groups had a higher risk than non-users. The study appears in the Jan. 26 issue of the Journal of the American Medical Association.

The researchers analyzed data from 46,000 women between 1973 and 1980 and found that the relative risk for breast cancer increased by 8% per year for the estrogen-progestin therapy compared to 1% per year for estrogen therapy in women who had used the hormones in the previous four years. There was no increase in risk among women who had stopped either therapy for more than four years. For women who had used hormone therapy during the previous four years, the average length of use was 3.6 years for estrogen-progestin compared to 10.3 years for estrogen alone.

When the scientists looked at subgroups of women, they found increases in risk associated with hormone replacement therapy among lean women, but not among heavy women. Compared to non-users, lean women experience a risk increase of 12% for each year of estrogen-progestin use and 3% for estrogen alone.