Introduction
Breast cancer has an enormous impact on the health of women. Approximately 180,000 women are diagnosed with breast cancer annually in the United States, and breast cancer accounts for approximately 30% of all incident cancers among women. Each year, 44,000 women die of breast cancer, making it the second leading cause of cancer deaths among American women, after lung cancer, and the leading cause of death among women aged 40 to 55 years. Breast cancer is rare among men, with only 1,600 incident cases and 400 deaths estimated for the United States in 1998.1 The lifetime risk through age 85 years of being diagnosed with breast cancer for an American woman is approximately 1 in 8, or 12.5%, whereas the lifetime risk of dying from breast cancer is approximately 3.4%.

This section begins with a description of the marked variations in breast cancer rates among populations and over time. Decades of research have led to a substantial understanding of the factors involved in the development of breast cancer; known and suspected risk factors are reviewed and considered in relation to etiologic mechanisms leading to breast cancer. The contribution that known risk factors make to the existing variations in rates is considered; this contribution is central to the question of whether unidentified pollutants or dietary factors explain the present high rates in the United States. Because of the major investments in breast cancer research, the means for preventing a substantial fraction of breast cancer now exist; strategies that can be adopted by individual women, their health care providers, and societies and governments as a whole are examined.

Walter C. Willett, Beverly Rockhill, Susan E. Hankinson, David J. Hunter and Graham A. Colditz

W. C. Willett: Harvard Medical School, Boston, Massachusetts; Department of Nutrition, Harvard School of Public Health, Boston, Massachusetts
B. Rockhill: Harvard Medical School, Brigham and Women’s Hospital, Boston, Massachusetts
S. E. Hankinson: Departments of Medicine and Epidemiology, Harvard Medical School and Harvard School of Public Health, Boston Massachusetts
D. J. Hunter: Departments of Epidemiology and Nutrition, Harvard School of Public Health, Boston, Massachussetts
G. A. Colditz: Department of Medicine, Harvard Medical School, Boston, Massachussetts

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