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About WHI

About WHIMosaic of a Woman's Face


The Women's Health Initiative (WHI) is a long-term national health study that has focused on strategies for preventing heart disease, breast and colorectal cancer, and osteoporotic fractures in postmenopausal women. These chronic diseases are the major causes of death, disability, and frailty in older women of all races and socioeconomic backgrounds.

This multi-million dollar, 20+ year project, sponsored by the National Institutes of Health (NIH), National Heart, Lung, and Blood Institute (NHLBI), originally involved 161,808 women aged 50-79 at recruitment, and was one of the most definitive, far-reaching clinical trials of post-menopausal women's health ever undertaken in the U.S. The WHI Clinical Trial and Observational Study focused on many of the inequities in women's health research and will continue to provide practical information to women and their physicians about hormone therapy, dietary patterns, calcium/vitamin D supplementation, and their effects on the prevention of heart disease, cancer and osteoporotic fractures.

The WHI had two major parts: a randomized Clinical Trial and an Observational Study. The randomized controlled Clinical Trial (CT) enrolled 68,132 postmenopausal women between the ages of 50-79 into trials testing three prevention strategies. If eligible, women could choose to enroll in one, two, or all three of the trial components. The components are:

  • Hormone Therapy Trials (HT): This component examined the effects of combined hormones or estrogen alone on the prevention of coronary heart disease and osteoporotic fractures, and associated risk for breast cancer. Women participating in this component took hormone pills or a placebo (inactive pill) until the Estrogen plus Progestin and Estrogen Alone trials were stopped early in July 2002 and March 2004, respectively. All HT participants continued to be followed without intervention until close-out.

  • Dietary Modification Trial (DM): The Dietary Modification component evaluated the effect of a low-fat and high fruit, vegetable and grain diet on the prevention of breast and colorectal cancers and coronary heart disease. Study participants followed either their usual eating pattern or a low-fat dietary pattern.

  • Calcium/Vitamin D Trial (CaD): This component began 1 to 2 years after a woman joined one or both of the other clinical trial components. It evaluated the effect of calcium and vitamin D supplementation on the prevention of osteoporotic fractures and colorectal cancer. Women in this component took calcium and vitamin D pills or a placebo.

The Observational Study (OS) is examining the relationship between lifestyle, health and risk factors and specific disease outcomes. This component involves tracking the medical history and health habits of 93,676 women. Recruitment for the observational study was completed in 1998 and participants were followed for 8 to 12 years.

The Fred Hutchinson Cancer Research Center in Seattle, WA serves as the WHI Clinical Coordinating Center for data collection, management, and analysis of the WHI. The WHI Clinical Trial and Observational Study were conducted at 40 Clinical Centers nationwide. Recruitment began in September 1993 and continued through October 1998 for the CT. The OS enrollment continued through December 1998. Close-out of the WHI CT occurred between October 2004 and March 2005.

WHI Extension Studies continued follow-up of consenting participants, the first consenting participants from each of the original WHI study components for an additional five years (2005-2010) of follow-up, and the second consenting participants from the first Extension Study for an additional five years (2010-2015). In 2015, WHI was awarded additional funding to continue follow-up of participants through 2020. Annual updates on health outcomes are collected by mail from the participants enrolled in each Extension Study. See WHI Timeline for WHI and the Extension Studies.

The following sections describe the study's design, procedures, and components:

Related Publications

Baseline monograph