The relation of noncontraceptive estrogen use to epithelial ovarian cancer was evaluated in a case-control study conducted in hospitals mainly in the northeastern United States. There were 377 cases diagnosed within the year before hospital admission and 2,030 hospital controls; data were collected by interview in the hospital. Compared with women who never took noncontraceptive estrogens, the overall relative risk estimate for women whose estrogen use lasted at least one year and was not combined with progestogens or testosterone was 1.2 (95% confidence interval (CI) 0.8-1.9), after taking into account risk factors for ovarian cancer. There were 55 cases of the endometrioid, clear cell, or malignant mixed mesodermal cell type; the corresponding relative risk estimate was 0.9 (95% CI 0.3-3.0). There were 26 cases of undifferentiated cell type, with a relative risk estimate of 3.6 (95% CI 1.2-11). Relative risk estimates were similar in a subset of the cases (57%) for which pathology slides were reviewed. For estrogen use of long duration, use of high-dose preparations, or use in the distant past, the relative risk estimates were not significantly different from 1.0. The estimates were elevated for some categories of use, but not consistently--for example, for an interval of 5-9 years since estrogen use began (relative risk (RR) = 2.7), but not after shorter or longer intervals, and for use of conjugated estrogens with a dose of 0.3 mg (RR = 3.2) or 1.25 mg (RR = 2.4), but not for doses of 0.625 mg or 2.5 mg. The relative risk estimate was also elevated for use by nulliparous women (RR = 2.4). The results suggest that, overall, noncontraceptive estrogen use is not associated with the risk of epithelial ovarian cancer. Furthermore, our data do not support the hypothesis that estrogens increase the risk of endometrioid ovarian cancer. The elevated estimates could be due to multiple stratification of the data, but they should be explored in further studies, given the lethality of ovarian cancer and the common use of estrogens by postmenopausal women.