According to the New England Journal of Medicine:

the age-adjusted incidence rate of breast cancer in women in the United States fell sharply (by 6.7%) in 2003, as compared with the rate in 2002. Data from 2004 showed a leveling off relative to the 2003 rate, with little additional decrease. Regression analysis showed that the decrease began in mid-2002 and had begun to level off by mid-2003. A comparison of incidence rates in 2001 with those in 2004 (omitting the years in which the incidence was changing) showed that the decrease in annual age-adjusted incidence was 8.6% (95% confidence interval [CI], 6.8 to 10.4)…The decrease in breast-cancer incidence seems to be temporally related to the first report of the Women’s Health Initiative and the ensuing drop in the use of hormone-replacement therapy among postmenopausal women in the United States. The contributions of other causes to the change in incidence seem less likely to have played a major role but have not been excluded.

A standard puzzle in health care economics is that groups of people who receive more medical services do not show better health outcomes than comparable groups receiving fewer services. Robin Hanson argues that this suggests that medical services are as likely to make you worse as make you better. The women who used to receive hormone therapy and subsequently got breast cancer might agree.