The relation between the risk of breast cancer and the use of noncontraceptive estrogens was investigated in a hospital-based study of 1,610 women with breast cancer and 1,606 with other conditions. The overall relative risk estimate for conjugated estrogens first taken at least 18 months before admission, compared with never-use of any noncontraceptive estrogens, was 0.9 (95% confidence interval, 0.7 to 1.1). For other estrogens first taken at least 18 months before admission, the estimate was 0.8 (0.6 to 1.1). The results were similar when known risk factors for breast cancer were taken into account. Among postmenopausal women, conjugated estrogens did not appear to increase the risk of breast cancer, even when taken for many years or in the distant past. There was no evidence of an increased risk due to conjugated estrogen use among subgroups of women defined according to various risk factors for breast cancer. The results of this study suggest that noncontraceptive estrogens do not increase the risk of breast cancer.