Prospective study of estrogen replacement therapy and risk of breast cancer in postmenopausal women [published erratum appears in JAMA 1991 Apr 10;265(14):1828] 
 We prospectively examined the use of estrogen replacement therapy in relation to breast cancer incidence in a cohort of women 30 to 55 years of age in 1976.
 During 367 187 person-years of follow-up among postmenopausal women, 722 incident cases of breast cancer were documented.
 Overall, past users of replacement estrogen were not at increased risk (relative risk, 0.98; 95% confidence interval, 0.81 to 1.18), including even those with more than 10 years since last [corrected] use (relative risk after adjustment for established risk factors, 0.70; 95% confidence interval, 0.45 to 1.10).
 However, the risk of breast cancer was significantly elevated among current users (relative risk, 1.36; 95% confidence interval, 1.11 to 1.67).
 Among current users, a stronger relationship was observed with increasing age but not with increasing duration of use.
 These data suggest that long-term past use of estrogen replacement therapy is not related to risk of breast cancer but that current use may modestly increase risk.
