Women’s Health Initiative (WHI)

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What is the goal of the WHI?

The Women's Health Initiative (WHI), sponsored by the National Heart, Lung, and Blood Institute (NHLBI), is a long-term national health study that focuses on strategies for preventing heart disease, breast and colorectal cancer, and osteoporosis in postmenopausal women. These chronic diseases are the major causes of death, disability, and frailty in older women of all races and socioeconomic backgrounds.

The original WHI study had three parts—a clinical trial, an observational study, and a community prevention study—and completed data collection in 2005. The WHI continues to contribute to the science of women’s health through extension and ancillary studies.

WHI extension studies collect long-term data from WHI participants to complement the original WHI study. The current extension study is collecting annual health information from consenting WHI participants through 2020, focusing on cardiovascular events and aging.

WHI ancillary studies are separate research projects that enroll WHI participants. Examples include:

  • The Women’s Health Initiative Strong and Healthy Study (WHISH), which examines the health benefits of a physical activity program in older women;
  • The Objective Physical Activity and Cardiovascular Health Study (OPACH), which used wearable devices to measure the effect of physical activity on cardiovascular health in older women; and
  • The Women's Health Initiative Sleep Hypoxia Effects on Resilience (WHISPER), which examines whether sleep-disordered breathing and the resulting low levels of oxygen in the blood are associated with an increased risk of cardiovascular events, including heart attack, stroke, heart failure, cancer, and cognitive decline.


  • The WHI is one of the largest women's health projects ever launched in the United States, having enrolled more than 161,000 women at 40 clinical centers.
  • The WHI randomized controlled clinical trial enrolled more than 68,000 postmenopausal women between the ages of 50 and 79.
  • The WHI observational study tracked the medical histories and health habits of more than 93,000 women, providing information to complement the clinical trial.
  • The WHI found that hormone replacement therapy did not prevent heart disease in postmenopausal women as once thought.
  • The scientific knowledge gained from the WHI hormone trials has helped save an estimated $35.2 billion in direct medical costs in the United States.

How does the WHI contribute to scientific discoveries?

The WHI and its findings have changed women’s health and medical practice around the world by helping women and their health care providers make more informed decisions, particularly about the use of hormone therapy after menopause.

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WHI Hormone Therapy Findings
The WHI demonstrated that the use of estrogen plus progestin hormone therapy after menopause increased the risk for heart disease, stroke, blood clots, breast cancer, and dementia. Although hormone therapy with estrogen alone had some benefits for younger women who had a prior hysterectomy, estrogen increased the risk for stroke and blood clots in these and other women in the study.

From the WHI, we now know that hormone therapy—estrogen plus progestin or estrogen alone—should not be used in postmenopausal women to prevent heart disease or to lower cholesterol levels. Hormone therapy is still an option for some women to help relieve moderate to severe symptoms that occur early in menopause.

Other Findings
The WHI dietary modification trial found that a low-fat diet did not significantly reduce the risk of breast cancer, heart disease, or stroke, and did not reduce the risk of colorectal cancer. A low-fat diet did reduce the risk of ovarian cancer.

The WHI calcium/vitamin D trial showed that calcium and vitamin D supplements provide a modest benefit in preserving bone mass and preventing hip fractures in certain groups, including older women. The supplements do not prevent other types of fractures or colorectal cancer.

Contributing to Future Discoveries
The WHI continues to yield new insights that advance the understanding of women’s heart disease and other diseases. In 2016, the NHLBI released its Strategic Vision, which will guide the Institute’s research activities for the coming decade. Data from the WHI will help researchers address many of the objectives, compelling questions, and critical challenges identified in the plan. For example, WHI data may help researchers understand the unique health concerns of women, as well as health differences between women of different ages, races, ethnicities, and socioeconomic backgrounds. This understanding may help researchers develop more targeted treatment and prevention strategies and reduce health disparities.

The NHLBI Trans-Omics for Precision Medicine (TOPMed) program is leveraging biospecimens and data from the WHI and other large population studies to advance research on precision medicine. Precision medicine is an emerging approach to disease that considers the unique genes and environment of each patient to develop tailored prevention and treatment strategies.

Investigators can access WHI data resources for other research projects. A subset of WHI data are available through the NHLBI’s Biologic Specimen and Data Repositories Information Coordinating Center (BioLINCC). Phenotypic and genetic data from some WHI participants are available through the Database of Genotypes and Phenotypes (dbGaP).

Read Advancing Women's Heart Health to learn more.

How was the WHI conducted?

The original WHI study included a clinical trial, an observational study, and a community prevention study. The clinical trial and observation study were conducted at 40 clinical centers nationwide and enrolled more than 161,000 women. A coordinating center, at the Fred Hutchinson Cancer Research Center, managed data collection and analysis. The community prevention study was conducted at eight university-based centers.

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The WHI Clinical Trial
The WHI clinical trial enrolled more than 68,000 postmenopausal women between the ages of 50 and 79. The WHI clinical trial included three separate randomized controlled trials. In randomized controlled trials, participants are assigned by chance into separate groups and neither the researchers nor the participants choose which group.

If eligible, WHI clinical trial participants could choose to enroll in one, two, or all three of these trials.

  • The hormone trial had two studies: the estrogen-plus-progestin study of women with a uterus and the estrogen-alone study of women without a uterus. The studies looked at the effect of hormone therapy on the prevention of heart disease and osteoporosis, and any associated risk for breast cancer. In both studies, women took hormone pills or a placebo—an inactive pill.
  • The dietary modification trial studied the effect of a diet low in fat and high in fruits, vegetables, and grains on the prevention of breast and colorectal cancer and heart disease. Study participants followed either their usual eating pattern or a low-fat eating program.
  • The calcium/vitamin D trial started up to two years after a woman joined one or both of the other studies. It looked at the effect of calcium and vitamin D supplements on the prevention of osteoporosis-related fractures and colorectal cancer. Women in this component took calcium and vitamin D pills or a placebo.

The WHI Observational Study
The observational study tracked the medical histories and health habits of more than 93,000 postmenopausal women between the ages of 50 to 79. The study followed participants for an average of eight years and was conducted at the same time as the WHI clinical trial.

Women who joined this study filled out periodic health forms and visited the clinic three years after enrolling in the study. Participants were not required to take any medicine or change their health habits. The study followed the women's health over a long period of time and provided information that complemented the WHI clinical trial.

The WHI Community Prevention Study
The Centers for Disease Control and Prevention and the National Institutes of Health partnered on the WHI community prevention study, which aimed to develop model programs to encourage women of all races and socioeconomic backgrounds to adopt healthy behaviors, such as improving diet, quitting smoking, and increasing physical activity. Eight university-based prevention centers conducted and evaluated model programs.