Breast cancer risk factors include age; a family history of the disease in first-degree relatives (particularly if premenopausal with bilateral disease); proliferative benign breast disease with or without atypia; mammographic parenchymal pattern showing glandular, dense, dysplastic, or Wolfe P2 changes; and obesity. Women in these risk groups have annual breast cancer incidence rates 2 to 10 times higher than baseline. Using data from the National Cancer Institute SEER Program, the U.S. 1987 census estimates, and published prevalence and incidence estimates for breast cancer risk factors, estimates were made for the number of women in the white female population who are at increased risk for breast cancer and who might serve as suitable subjects for an antiestrogen chemoprevention intervention trial. More than 30 million white women are older than 50 years. Two million women older than 50 years have at least one first-degree relative with breast cancer. Six million women over age 50 have undergone breast biopsy for benign disease; one-fourth of these women have proliferative changes, and 11% also have a family history of breast cancer. More than 8 million women older than 50 years are obese, and at least a million older women have high-risk mammographic parenchymal patterns. Thus, there are at least 12 million women at increased risk for breast cancer in the United States, and each year 200,000 additional women enter the high-risk pool. These data indicate that sufficient numbers of women at increased risk for breast cancer are present in the population to justify a chemoprevention trial. The optimal recruitment strategy is yet to be identified.