LONG-TERM POSTMENOPAUSAL ESTROGEN THERAPY MAY BE ASSOCIATED WITH INCREASED RISK OF BREAST-CANCER - A COHORT STUDY

Reports of a role of postmenopausal estrogen replacement therapy in th e development of breast cancer have been inconsistent. Although many e pidemiologic studies have failed to show an association between short- term use of estrogen and breast cancer, there are indications that lon g-term use may present an increased risk. We undertook a long-term, re trospective cohort study of the incidence of breast cancer in women wh o 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 us ers developed breast cancer, as did 9 (4.1%) of the nonusers; the rela tive risk (RR) for estrogen use was 2.8 [95% confidence interval(95% C I) 1.3-5.9]. Adjustment for age and multiple breast cancer risk factor s, including breast cancer surveillance, reduced the RR for estrogen t o 2.0 (95% CI 0.9-4.5). We conclude that long-term estrogen use is ass ociated with a substantially increased risk of breast cancer.