Breast cancer accounts for one-third of cancer diagnoses and 15% of cancer deaths in U.S. women. Its 192,000 cases and 40,000 deaths in 2001 make it the most common incident cancer (excluding superficial skin cancers) and second leading cause of cancer death. Over one-half of the 300,000 breast cancer deaths worldwide in 1990 (the latest year with such data) occurred in developed countries, but annual mortality rates ranged from 27/100,000 women in northern Europe to 4/100,000 women in Asia. Incidence data are less complete, although 1988-1992 rates varied threefold: low in Asia, intermediate in South America and Eastern Europe, and high in North America and Western Europe. Migrant studies suggest that lifestyle factors largely explain these international differences. U.S. incidence rates are generally 20%-40% higher in white women than in non-white women, but are higher in young (under age 40) black women than in young white women. Incidence rates rose in the 1970s, leveled off in the 1990s, and are declining for young women. Women in some areas of the northeast U.S. have twofold higher mortality than that of other U.S. women, but reproductive and socioeconomic characteristics explain much of that difference. In the 1970s and 1980s, mortality rates held steady in developed countries but rose in developing countries. Since 1987 mortality rates fell by 25% as a result of earlier detection and improved treatment. Age-period-cohort analyses indicate that changes in recognized risk factors may affect mortality patterns. Continued analysis of international and intranational trends may reveal targets for multidisciplinary intervention and prevention efforts.