Declines in breast cancer after the WHI: apparent impact of hormone therapy.

Large numbers of US women stopped taking hormone therapies (HT), especially estrogen/progestin (EP) formulations, after the Women's Health Initiative trial detected elevated risks of breast cancer in EP users and was halted in July 2002. Recent reports have indicated substantial and significant declines in population-based breast cancer incidence, particularly hormone-sensitive forms, for 2003 and 2004. Are these events linked? This commentary considers the available evidence linking the mass cessation of HT in 2002 to the breast cancer incidence declines of 2003/2004 and quantifies the potential impact of the cessation on the overall burden of breast cancer in the US.