The Women's Health Study (WHS) is a randomized, double-blind, placebo-controlled trial using a 2x2 factorial design. The WHS is sponsored by both NHBLI (HL080467) and NCI (CA047988). Approximately 1.75 million female health professionals were contacted by mail to determine if they were suitable for inclusion in the study. A three-month run-in phase was performed to screen out those with poor compliance. Randomization, which began in February 1993 and ended in January 1996, was stratified on five-year age groups. A total of 39,876 participants were randomly assigned to either Vitamin E (600 IU every other day) or placebo; and to aspirin (100 mg every other day) or placebo. IN the 2x2 factorial design, women were randomly assigned to active aspirin and placebo vitamin E (n=9,968), placebo aspirin and active vitamin E (n=9,971), active aspirin and active vitamin E (n=9,966), or placebo aspirin and placebo vitamin E (n=9,971). A description of the characteristics of women in these 4 groups is provided in J Women's Health Gend Based Med 2000;9:19-27. In the main analyses, all women on active aspirin (n=19,934) were compared to women on placebo aspirin (n=19,942); and all women on active vitamin E (n=19,937) were compared to women on placebo aspirin (n=19,939).