As life expectancy increases and members of the postwar generation settle into their fifth decade of life, hormone replacement therapy--estrogen or an estrogen-progestin combination--has become a major research interest. An extensive, but often confusing and even contradictory, literature exists on the uses of hormone replacement for the treatment and prevention of a multitude of difficulties that may be associated with the perimenopausal and postmenopausal periods. These include hot flushes, vaginal changes, urinary tract changes, changes in sexuality, affective or emotional symptoms, changes in the oral mucosa and skin, loss of memory and Alzheimer's disease, bone loss and osteoporosis, and cardiovascular disease. This article reviews the literature in each of these areas. It also reviews studies relating to possible side effects of hormone therapy, including endometrial cancer, gall bladder disease, and breast cancer. The article outlines principles for practitioners to follow in assisting women to make informed and individualized decisions about this therapy. Part II of this article, which will appear in the May/June 1996 issue of the Journal of Nurse-Midwifery, will cover specific therapeutic regimens and their management, as well as alternative therapies and preventive measures.