The number of breast cancer cases in postmenopausal American women plunged seven percent in 2003, a year after millions of women ditched the hormones after a major study linked them to a higher risk of heart, stroke and breast cancer.
The numbers continued to level off in 2004.
For some scientists, the numbers clearly showed a link between breast cancer and hormone replacement therapy (HRT), but others suggested that the trend could be explained by a relatively modest decline in the numbers of women getting mammograms in recent years.
To determine what was driving the statistic -- a backlash against HRT or lower detection rates -- researchers at the University of San Francisco, California, decided to review breast cancer rates among women who had been routinely screened over a period of several years.
The investigators reviewed the medical records of more than 232,000 women from seven mammography registries across the country who had been screened for breast cancer between 1997 and 2003.
The results clearly showed that breast cancer rates went down as the post-menopausal women abandoned their estrogen and progestin supplements.
The records showed that seven percent of the women in the study group quit using the hormones per year between 2000 and 2002 and a whopping 34 percent followed suit in each of the following two years. Over the same period, breast cancer cases declined annually by five percent.
"We think our results are pretty definitive and they point to hormones as the reason for the decline in breast cancer rates," said lead author Karla Kerlikowske, an epidemiologist at the University of San Francisco.
The paper appears in the Journal of the National Cancer Institute.