In women, dietary modification and life style affect the risk of breast cancer and may alter the hormonal status, while in experimental animals diet can alter the incidence of induced mammary tumors. In this study, a high fat diet increased the incidence of DMBA-induced tumors in rats while this increase in incidence was lowered and the effect of a high fat diet obliterated by an anti-prolactin during CB154. Premenopausal Japanese women had a higher estradiol level than their Caucasian counterpart. In Japanese but not Caucasian breast cancer patients, the estradiol decreased. When nurses were transferred from the Western to a vegetarian diet, the menstrual cycle was shortened, while their prolactin and testosterone decreased. Data indicate that dietary factors influence tumor incidence and hormone profile in rat mammary cancer and the hormonal status in women.