Objective: This study combines population changes in hormone therapy (HT) use with reported relative risks to estimate directly the impact of reductions in HT use on US breast cancer incidence. Methods: Using breast cancer incidence rates and prevalence estimates of HT use, the breast cancer incidence in HT users and non-users was derived. Attributable fraction calculations used risk data from the Million Women Survey (MWS), Collins' meta-analysis (CMA), and the initial or later data from the Women's Health Initiative (WHI) study. Results: Between 2000 and 2005, HT use fell from 25.0 to 11.3%. The reported breast cancer incidence (in women aged 40-79) fell 8.8%. Derived incidence rates among non-users of HT remained unchanged (MWS or later WHI data) or slightly lower (initial WHI and CMA data), suggesting reductions in incidence are almost entirely (MWS or later WHI data) or partially (initial WHI or CMA data) due to reduced HT use. Conclusion: Changes in reported breast cancer incidence may be partially or largely explained by changes in HT use in the US population.