Women’s Health Initiative (WHI)

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Current study runs through 2026
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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 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. The WHI is the largest women’s health prevention study ever conducted. 

WHI beyond the original study 

The initiative’s extension studies collect long-term data from 52,068 WHI volunteers to complement the original WHI study. The current extension study collects annual health information from WHI volunteers who agree to take part through 2026, with a focus on heart disease, cardiovascular events, and aging. 

Ancillary studies, on the other hand, are separate research projects that reach out to and enroll WHI volunteers. 

  • The Long Life Study (LLS) includes a subset of 7,875 people who will have clinical measurements and donate samples (biospecimens) with a 10-year follow-up assessment.
  • The Women’s Health Initiative Strong and Healthy Study (WHISH) looks at the health benefits of a physical activity program for older women;
  • The Objective Physical Activity and Cardiovascular Health Study (OPACH) used wearable devices to measure how physical activity impacts cardiovascular health in older women. 
  • The Women's Health Initiative Sleep Hypoxia Effects on Resilience (WHISPER) studies whether sleep-disordered breathing such as sleep apnea and the low levels of oxygen in the blood that are caused by it are related to more risk of diseases such as 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, with more than 161,000 women enrolled 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 medical histories and health habits of more than 93,000 women to add information that complements the clinical trial.
  • The WHI found that hormone replacement therapy did not prevent heart disease in post-menopausal women as was once thought.
  • • The WHI hormone trials added important information that helped save about $35.2 billion in direct medical costs in the United States.

How does the WHI contribute to scientific discoveries?

The Women’s Health Initiative (WHI) and its findings have changed women’s health and how medicine is practiced around the world. Information from the WHI helps women and their healthcare providers make more informed decisions, particularly about the use of hormone therapy after menopause. 

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WHI Hormone Therapy Findings
The WHI showed that using estrogen plus progestin hormone therapy after menopause raised the risk of heart disease, stroke, blood clots, breast cancer, and dementia. 

Hormone therapy with estrogen alone had some benefits for younger women who had a prior hysterectomy. However, estrogen raised the risk of stroke and blood clots for these women and others in the study.

This initiative helped establish 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 early in menopause to help relieve moderate to severe symptoms.

Other findings
The WHI dietary modification trial found that a low-fat diet did not have a significant impact on a woman’s 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 initiative’s calcium/vitamin D trial showed that calcium and vitamin D supplements provide a modest benefit to preserve bone mass and prevent hip fractures for some people, including older women. These supplements do not prevent other types of fractures or colorectal cancer.

Contributing to future discoveries
The WHI continues to bring new insights to what we know about women’s heart disease and other diseases. Data from the WHI helps researchers meet NHLBI’s Strategic Vision objectives, answer important and compelling questions, and tackle critical challenges laid out in that plan. 

For example, WHI data may help researchers understand unique health concerns for women as they age. The data may also provide important details about health differences between women of different ages, races, ethnicities, and backgrounds. Understanding these differences may help researchers create more targeted treatment and prevention strategies and work to lower health disparities.

The NHLBI Trans-Omics for Precision Medicine (TOPMed) program uses samples (biospecimens) and data from the WHI and other large population studies for precision medicine research. Precision medicine considers the genes and environment unique to each person, with the goal of developing personalized prevention and treatments.

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).

Advancing Women's Heart Health has more information about NHLBI’s long history of including women in cardiovascular studies and how this research increases understanding of heart disease’s unique impacts on women. 

How was the WHI conducted?

The original Women’s Health Initiative (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 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 volunteers 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 as well as any associated risk of breast cancer. In both studies, women took hormone pills or a placebo—an inactive pill.
  • The dietary modification trial studied whether a diet low in fat and high in fruits, vegetables, and grains had an effect on whether someone developed breast and colorectal cancer and heart disease. Study participants followed 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 trial 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 people 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 health forms periodically 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 each woman’s health over a long period of time. The findings 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.