WHI is comprised of a program office at the National Heart, Lung, and Blood Institute, National Institutes of Health (NIH), 40 clinical sites at which the participants are seen, a clinical coordinating center at the Fred Hutchinson Cancer Center in Seattle, and other supporting academic and laboratory units. The study design has been described in detail (Women’s Health Initiative Study Group, 1998). Basically, from 1993 to 1997, postmenopausal women ages 50-79 were recruited into a set of three large, preventive, randomized, placebo-controlled trials, listed as follows with the primary outcomes: (1) estrogen alone or estrogen plus progestin therapy to assess the effect on coronary heart disease; (2) low-fat diet to prevent incident breast cancer; and (3) calcium and vitamin D dietary supplements to prevent clinical fractures. The last trial, the calcium/vitamin D intervention, was a superimposed randomization on the participants of the other two trials and did not include separate participants. Of note, that makes the interpretation of the outcomes a bit more complex, despite the randomized experimental design, because the impact of the other intervention(s) must be accounted for in all analyses of outcomes.

Recruitment to WHI was conducted by the 40 participating clinical sites, using such methods as media advertising, direct population mailing, community lectures and public service announcements, and celebrity endorsements. This yielded about 353,000 women who were evaluated for participation in WHI. About 68,000 enrolled in the trials. Of the remainder, those who were ineligible, unwilling, or unable to participate in the clinical trials were invited into a cohort study, the Observational Study (OS), in which no interventions took place; about 93,000 women began full participation in the OS. There were many specific reasons why potential trial participants opted for OS participation. Three of the most important were not being able to spend the time undergoing the trials’ rigors; unwillingness to be randomized with respect to the use of hormonal therapy; and having too low a percentage of dietary fat calories, making them ineligible for the dietary modification trial.

Participants in the clinical trials as well as the OS had the same, extensive baseline data collection, including medical history, risk factors for important chronic illnesses, social and behavioral characteristics, mental health characteristics, and diet and dietary supplement use. In addition, and important to this discussion, virtually all participants had blood obtained through venipuncture, which was aliquoted and stored, and much of it is available today for additional studies, in addition to the many ancillary biomarker studies that have already taken place. Additional blood samples were obtained from some participants at selected intervals after the study onset. Of interest, aside from some planned safety