The relation between replacement estrogens and breast cancer in menopausal women (age 45–64 years) was evaluated in the Group Health Cooperative of Puget Sound, Washington State, a prepaid health care organization with fully computerized information on outpatient drug use and hospital diagnoses. Little association between current estrogen use and breast cancer was noted in women with a previous hysterectomy (relative risk (RR) = 1.1; 90% confidence interval (Cl), 0.7–1.8). In women with a natural menopause there was a positive association between current estrogen use and breast cancer (RR = 3.4; 90% Cl, 2.1–5.6). This association was strong in naturally menopausal women aged 45–54 years (RR = 10.2) and weaker in the older age group (RR = 1.9).
