The growth of breast cancer may be mediated by endogenous or exogenous sex steroid hormones, particularly estrogen. However, neither contraceptive nor noncontraceptive estrogen use has been associated definitively with an increased risk of developing breast cancer. In this study, we addressed a corollary question: If a postmenopausal woman develops carcinoma of the breast, is her survival affected by previous use of replacement estrogen? Two hundred fifty-six postmenopausal women with breast cancer entered our Tumor Registry between 1972-1981, inclusive. Of these, 174 took no replacement estrogen before the diagnosis (never-users), 21 had used estrogen previously (past users), and 61 were taking estrogen at the time of diagnosis (current users). Survival analysis revealed a median survival of less than 84 months after diagnosis for never- and past users and greater than 143 months for current users, but these differences were not significant when controlled for stage of disease at diagnosis. We conclude that prior postmenopausal estrogen replacement therapy does not compromise survival in women who subsequently develop carcinoma of the breast.