Long-term postmenopausal estrogen therapy may be associated with increased risk of breast cancer a cohort studyEttinger, Bruce1; Quesenberry, Charles1; Schroeder, David A.2; Friedman, Gary1Menopause: November 2018 - Volume 25 - Issue 11 - p 1191–1194 doi: 10.1097/GME.0000000000001216 Commemorative Papers Abstract Author InformationAuthors Article MetricsMetrics Reports of a role of postmenopausal estrogen replacement therapy in the development of breast cancer have been inconsistent. Although many epidemiologic studies have failed to show an association between short-term use of estrogen and breast cancer, there are indications that long-term use may present an increased risk. We undertook a long-term, retrospective cohort study of the incidence of breast cancer in women who had taken long-term estrogen (average 17.2 years), compared to women who had not taken estrogen. Subjects were 454 women born between 1900 and 1915, who were members of a large health maintenance organization in northern California. By the end of 1995, 26 (11.2%) of estrogen users developed breast cancer, as did 9 (4.1%) of the nonusers; the relative risk (RR) for estrogen use was 2.8 [95% confidence interval (95% CI) 1.3-5.9]. Adjustment for age and multiple breast cancer risk factors, including breast cancer surveillance, reduced the RR for estrogen to 2.0 (95% CI 0.9-4.5). We conclude that long-term estrogen use is associated with a substantially increased risk of breast cancer. 1Kaiser Permanente Medical Care Program, Division of Research, Oakland, California 2Portland, Oregon. Address correspondence to: Dr. Bruce Ettinger, Division of Research, Kaiser Permanente Medical Care Program, 77th floor, Oakland, CA 94611, U.S.A. Received 12 April, 1997 Accepted 1 July, 1997 © 2018 by The North American Menopause Society.