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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.

Both animal experiments and certain well-established breast cancer risk factors suggest that risk to the disease is fundamentally determined by the hormones of the pituitary-gonadal axis. Although international comparisons of urinary estrogens have given support to this ypothesis, case-control studies and international comparisons of plasma estrogens and prolactin have not. Methodological problems and sampling biases probably account for the inconsistency of these investigations. Taking advantage of the known familial increased risk to breast cancer, we conducted comparative studies of teenage daughters of patients with breast cancer, including a group of girls whose mothers had bilateral breast cancer when they were less than 50 years old. The results of these studies revealed that these high-risk girls appear to have elevated levels of estrogens, prolactin, and progesterone.

A case-control study was conducted in Los Angeles County, California, of 163 very young breast-cancer cases (all aged 32 or less at diagnosis) to investigate the role, if any, of oral contraceptives (OC) in the development of the disease. OC use before first full-term pregnancy (FFTP) was associated with an elevated risk, which increased with duration of OC use (relative risk approximately 2.2 at 6 years of use, P < 0.01). This increased risk could not be explained by other risk factors. OC use after FFTP was not associated with any change in risk. A first-trimester abortion before FFTP, whether spontaneous or induced, was associated with a 2.4-fold increase in breast-cancer risk (P < 0.005).

Breast-cancer incidence and mortality rates in different countries were found to be correlated with height, weight and age at menarche, all of which have been identified as risk factors in cohort or case-control studies of breast cancer. There were, however, correlations with total fat and animal protein consumption per capita even after controlling for the 3 anthropometric variables. This suggests that, while some of the effects of diet on breast-cancer rates may be mediated through effects on these known risk factors, there may be more direct effects as well.