The Women's Health Initiative (WHI) is an ongoing study of women and their health that is designed to evaluate heart disease, breast and colorectal cancer, and bone fractures in postmenopausal women. The WHI is supported by the National Heart, Lung, and Blood Institute of the U.S. National Institutes of Health.

Established in 1991, WHI is the first such study to examine the health of a very large number of women over a long period of time. More than 160,000 women, ages 50 to 79, from many different racial and ethnic backgrounds and 45 U.S. communities are participating in the study.

A major focus of the WHI is a trial of the long-term effects of postmenopausal hormone therapy on heart disease, osteoporosis, and colorectal and breast cancer.