The BWHS is an ongoing prospective cohort study of 59 000 black women residing in the United States, designed to examine risk factors for cancer and other chronic diseases.15,17 In 1995, letters of invitation and questionnaires were mailed to subscribers of Essence magazine, a popular magazine that targets black women, members of several professional organizations, and friends and relatives of early respondents. The cohort that has been followed comprises 59 000 black women aged between 21 and 69 years at baseline, whose addresses were valid a year after study entry. Participants originate from across the United States, with approximately 80% from California, Louisiana, Indiana, Georgia, Illinois, Maryland, Massachusetts, Michigan, New Jersey, New York, South Carolina, Virginia, and the District of Columbia. Follow-up questionnaires have been sent biennially, with follow-up averaging more than 80% of the original cohort through 2003. Demographic characteristics and information about medical history, behavioral habits, diet, medication use, physical activity, socioeconomic parameters, and reproductive history have been obtained. In 1995, 6.0% of the women reported a history of myocardial infarction, stroke, or angina, and 2.0% reported a history of cancer. The study was approved by the institutional review board of Boston University, Boston, Massachusetts.