Changing ratio of breast cancer incidence rates with age of black females compared with white females in the United States.

Age-standardized breast cancer rates were approximately 30% lower in U.S. black women compared to white women. This observation concealed the fact that black women under age 40 years had a higher incidence of breast cancer than did white women, whereas white women over 40 years had a higher incidence. The known risk factors for breast cancer development (early age at menarche, late age at first full-term delivery, and a late age of menopause) differed in black and white populations, which might explain this difference in breast cancer incidence between blacks and whites at different ages.