Evidence indicates that early life events and conditions, possibly extending to the intrauterine stages of life, and including energy restriction in early life, affect the risk of breast cancer. The mechanism of this effect is likely to be through a reduction in mammary gland mass and, inferentially, the total number of ductal stem cells. The evidence derives from epidemiologic and animal studies. It can explain certain epidemiologic findings that cannot be accounted for by more established breast cancer risk factors, including the more frequent occurrence of breast cancer in the left breast and the higher incidence of this disease among caucasian women than among Asian women in Asia.