A woman spends about one-third of her life in her postmenopausal years. Some women supplement this period of decreased estrogen production with estrogen replacement therapy (ERT). Since the 1970s, we have evaluated the long-term risks and benefits of ERT in one population of women, the Leisure World retirement community. ERT is the most effective method for preventing osteoporotic bone loss and fractures in postmenopausal women. In Leisure World, ERT reduced the risk of hip fractures about 50 %. The effect is greatest in long-term users but may be lost after discontinuation. Postmenopausal osteoporosis affects the bones of the jaws as well as other skeletal bones. Bone loss in the jaws may result in tooth loss. In Leisure World, estrogen users retain more natural teeth than nonusers. Cardiovascular disease is the leading cause of hospitalization and death in women. In Leisure World, ERT reduced the risk of fatal and nonfatal myocardial infarction, ischemic heart disease, other heart disease, and stroke by 20-40 %. The reduction is greatest in long-term and/or current users. ERT is effective in women with and without cardiovascular disease risk factors. A most feared aspect of aging is Alzheimer's disease. In Leisure World, women who had used ERT had a reduced risk of Alzheimer's disease. Risk both increaseng dose and decreased with increasing duration of use. Estrogen use, however, is not without risk. Unopposed estrogen increases risk of endometrial cancer. Risk increases with increasing years of use and remains high after discontinuation. The most important potential risk of ERT is breast cancer. In Leisure World, women who had used a total accumulated estrogen dose of 1500 mg or more had nearly twice the risk of breast cancer compared with nonusers. Short-term low-dose users showed no substantial increased risk. The Leisure World Study shows risks and benefits of ERT similar to other reports in the literature. For postmenopausal women generally, the benefits of ERT--preventing osteoporotic fractures, reducing heart disease, decreasing mortality, and possibly reducing risk of Alzheimer's disease-out-weigh the risks of endometrial and breast cancers. A woman must be fully informed of the risks and benefits of hormone therapy and play an important role in deciding whether to take hormones and which regimen to use.