Breast cancer incidence and mortality in England and Wales and the United States increased between 1950 and 1973, mainly in women aged between 45 and 64 years. These increases appeared to be partly cohort-specific, beginning with cohorts born around 1899, and partly cross-sectional, beginning in the mid-1960s. In both countries, cohort-specific decreases in fertility paralleled the cohort-specific increases in breast cancer rates and may, at least in part, have been reponsible for them. Changes in other factors, such as age at menarche and menopause, use of rauwolfia derivatives and oestrogens, consumption of fat and meat, and breast cancer treatment were considered in relation to the cross-sectional increases in breast cancer rates. On the evidence available, it was not certain that any of these could explain the breast cancer increases.