The growth of hormone-dependent human breast cancer is related to the activity of endogenous estrogens. The evidence for an etiological role of endogenous estrogens is still circumstantial. Life style and in particular dietary factors are held responsible for large geographic differences and time-trends in breast cancer incidence. The measurement of urinary estrogen metabolites and plasma estrogens has given no satisfactory explanation for the latter. The newly developed interest in the bioavailability of plasma sex steroids may offer a better understanding of the biology and epidemiology of breast cancer. Based on our observations on the relationship between plasma free fatty acids and estrogen-protein-binding and recently gained insight in the metabolic consequences of different types of body fat distribution we postulate that Western life style may act on breast cancer incidence through an influence on body fat distribution and resulting changes in sex steroid availability.