OUTLINE: This is a randomized, double-blind, placebo-controlled study. Participants are stratified by participating institution. Participants are randomly assigned to receive either oral tamoxifen or oral placebo every day for 5 years in the absence of breast cancer development or pregnancy. Participants are followed every 6 months for 5 years; then annually for 5 years. Peer Reviewed and Funded or Endorsed by Cancer Research UK