The 15% drop in breast cancer that occurred in 2003 held steady for 2004, probably because, as in 2003, about 38% fewer women used hormone replacement therapy (HRT) than in 2002 and prior. 

The main reasons for assuming the drop is associated with less HRT use are that the decrease in incidence is evident only in women 50 years of age or older and involves primarily estrogen-receptor–positive tumors. Furthermore, the decrease began in mid-2002 shortly after the release of the results of the randomized Women's Health Initiative trial that reported significant increases in the risks for coronary heart disease and breast cancer in women taking combination estrogen–progestin preparations.  

The 20 or so million fewer prescriptions written for HRT seems to have resulted in the first substantial drop in breast cancer incidence in more than 25 years; however, other causes, including changes in screening mammography, cannot be excluded.  