Influence of Estrogen Plus Progestin on Breast Cancer and Mammography in Healthy Postmenopausal Women

To determine the relationship among estrogen plus progestin use, breast cancer characteristics, and mammography recommendations. DESIGN, SETTING, AND PARTICIPANTS: Following a comprehensive breast cancer risk assessment, 16 608 postmenopausal women aged 50 to 79 years with an intact uterus were randomly assigned to receive combined conjugated equine estrogens (0.625 mg/d) plus medroxyprogesterone acetate (2.5 mg/d) or placebo from 1993 to 1998 at 40 clinical centers. Screening mammography and clinical breast examinations were performed at baseline and yearly thereafter. MAIN OUTCOME MEASURES: Breast cancer number and characteristics, and frequency of abnormal mammograms by estrogen plus progestin exposure.

In intent-to-treat analyses, estrogen plus progestin increased total (245 vs 185 cases; hazard ratio [HR], 1.24; weighted P