There is growing and consistent evidence that estrogen use in postmenopausal women is associated with a substantial reduction in the occurrence of cardiovascular disease. However, remarkably little is known about the biological mechanisms by which estrogen therapy may influence risk. Even less information is available on the cardiovascular effects of combined estrogen-progestin use. PEPI was not designed to test whether estrogen and estrogen-progestin therapy is efficacious in the prevention of cardiovascular disease, as a much larger trial with clinical disease outcomes is needed to answer that question. However, PEPI will provide critical evidence regarding the potential effectiveness of the various estrogen and estrogen/progestin regimens ni altering risk factors for cardiovascular disease in women. Detailed information on factors such as adherence, side effects, and general patient acceptability will also be ascertained. The main results from PEPI will provide the scientific community with information on the basic actions of estrogen and estrogen/progestin therapy on four biological systems believed to be causally associated with cardiovascular disease occurrence. Further, since the trial is designed to continue for 3 years, PEPI will be able to provide information on longer term as well as short-term effects on these systems. Finally, the results from PEPI should enable women and their physicians to select an optimal hormonal regimen, i.e., one that is acceptable, safe, and provides the most beneficial and least deleterious changes in cardiovascular and other risk factors.