The ongoing Women's Health Initiative (WHI) program has made several contributions toward a better understanding of lifestyle strategies to improve the health of postmenopausal women. This large study of more than 160,000 women comprises a set of (now completed) randomized controlled clinical trials testing whether hormone therapy prevents coronary heart disease, a low-fat dietary pattern prevents cancer, and calcium and vitamin D prevent hip fractures and colorectal cancer. Prior to the publication of the results of the WHI trials, each of these lifestyle strategies had been recommended for prevention of disease. An observational component of the initiative examines the risk factors for chronic diseases.