Background: Postmenopausal hormone therapy has both benefits and hazards, including decreased risks of osteoporosis and cardiovascular disease and an increased risk of breast cancer. Methods: We examined the relation between the use of postmenopausal hormones and mortality among participants in the Nurses' Health Study, who were 30 to 55 years of age at base line in 1976. Data were collected by biennial questionnaires beginning in 1976 and continuing through 1992. We documented 3637 deaths from 1976 to 1994. Each participant who died was matched with 10 controls alive at the time of her death. For each death, we defined the subject's hormone status according to the last biennial questionnaire before her death or before the diagnosis of the fatal disease; this reduced bias caused by the discontinuation of hormone use between the time of diagnosis of a potentially fatal disease and death. Results: After adjustment for confounding variables, current hormone users had a lower risk of death (relative risk, 0.63; 95 percent confidence interval, 0.56 to 0.70) than subjects who had never taken hormones; however, the apparent benefit decreased with long-term use (relative risk, 0.80; 0.67 to 0.96, after 10 or more years) because of an increase in mortality from breast cancer among long-term hormone users. Current hormone users with coronary risk factors (69 percent of the women) had the largest reduction in mortality (relative risk, 0.51; 95 percent confidence interval, 0.45 to 0.57), with substantially less benefit for those at low risk (13 percent of the women; relative risk, 0.89; 95 percent confidence interval, 0.62 to 1.28). Conclusions: On average, mortality among women who use postmenopausal hormones is lower than among nonusers; however, the survival benefit diminishes with longer duration of use and is lower for women at low risk for coronary disease.