1. Dietary-modification trial—To examine the effects of a low-fat diet (20% of daily calories from fat), increased fruit and vegetable consumption (5 or more servings per day), and increased grain consumption (6 or more servings per day) compared with usual diet on breast cancer, colorectal cancer, and heart disease in postmenopausal women. (The trial also looked at effects of this diet modification on Type 2 diabetes and ovarian cancer.)

  2. Hormone trial1—To examine the effects on coronary heart disease and osteoporotic fractures and associated risk of breast cancer of (1) estrogen plus progestin2 compared with placebo in postmenopausal women with a uterus, and (2) of estrogen alone3 compared with placebo in postmenopausal women without a uterus.

A prospective observational study was also conducted to produce reliable estimates of the extent to which known risk factors predict heart disease, cancers, and fractures; to identify “new” risk factors for these and other diseases in women; to compare risk factors, presence of disease at the start of the study, and new occurrences of disease during the WHI in all study components; and to create a future resource for identifying biologic indicators of disease, especially substances and factors found in blood.

A WHI Extension Study to encompass 5 years of additional followup (from 2005–2010) includes participants from both the clinical trial and observational WHI components. The purposes of the additional followup are to describe the longer-term effects of the original interventions, to document change in hormone use in participants from the hormone therapy trials, to expand the list of scientific questions that can be reliably addressed in the WHI, and to provide an infrastructure able to support additional investigations that require some of the unique features of a very large longitudinal study of postmenopausal women.

A community prevention study to develop carefully evaluated, model programs that could be implemented in a wide variety of communities throughout the United States was also conducted but will not be described here.

Participant Enrollment

Participants were recruited at 40 clinical centers around the United States over a 5-year period—September 1993–December 1998—with a planned average of 8 years of followup and 2 years for data analysis. A total of 161,808 generally healthy postmenopausal women 50–79 years old were enrolled.


The hormone trials have ended but the women who were enrolled in these trials participated in a followup phase, which concluded in 2010.


Consisting of 0.625 mg of conjugated equine estrogens plus 2.5 mg of medroxyprogesterone acetate.


Consisting of 0.625 mg of conjugated equine estrogens.

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