Prior hormone therapy and breast cancer risk in the Women's Health Initiative randomized trial of estrogen plus progestin.

Objectives: To assess the extent to which prior hormone therapy modifies the breast cancer risk found with estrogen plus progestin (E+P) in the Women's Health Initiative (WHI) randomized trial.

Methods: Subgroup analyses of prior hormone use on invasive breast cancer incidence in 16,608 postmenopausal women in the WHI randomized trial of E+P over an average 5.6 years of follow-up.