To examine the risk of breast cancer after noncontraceptive treatment with estrogen, we conducted a prospective study of 23,244 women 35 years of age or older who had had estrogen prescriptions filled in the Uppsala region of Sweden. During the follow-up period (mean, 5.7 years) breast cancer developed in 253 women. Compared with other women in the same region, the women in the estrogen cohort had an overall relative risk of breast cancer of 1.1 (95 percent confidence interval, 1.0 to 1.3). The relative risk increased with the duration of estrogen treatment (P = 0.002), reaching 1.7 after nine years (95 percent confidence interval, 1.1 to 2.7). Estradiol (used in 56 percent of the treatment periods in the cohort) was associated with a 1.8-fold increase in risk after more than six years of treatment (95 percent confidence interval, 0.7 to 4.6). No increase in risk was found after the use of conjugated estrogens (used in 22 percent of the treatment periods) or other types, mainly estriols (used in 22 percent of the treatment periods). Although the numbers of women were smaller, the risk of breast cancer was highest among the women who took estrogen and progestin in combination for extended periods. The relative risk was 4.4 (95 percent confidence interval, 0.9 to 22.4) in women who used only this combination for more than six years. Among women who had previously used estrogens alone, the relative risk after three years or more of use of the combination regimen was 2.3 (95 percent confidence interval, 0.7 to 7.8). We conclude that in this cohort, long-term perimenopausal treatment with estrogens (or at least estradiol compounds) seems to be associated with a slightly increased risk of breast cancer, which is not prevented and may even be increased by the addition of progestins.