(1) Low levels of dietary calcium and vitamin D, biochemically interrelated, increase the promoting action of high dietary fat on chemically induced mammary carcinogenesis in animal studies. (2) High dietary fat increases mammary epithelial cell proliferation, particularly the "hormonally driven" hyperproliferation during breast growth and development in young animals. Increased dietary calcium (and probably vitamin D) lessens the increase of proliferation induced by high fat. These data, although limited, suggest that the maximum effect of diet (high fat increase, as well as calcium and vitamin D modulation) on eventual breast cancer may be during puberty, and adolescence, when the mammary gland is actively growing and developing. (3) An inverse epidemiological correlation has been developed between sunlight availability as a source of vitamin D and the risk of breast cancer in the U.S. and Canada. (4) Current vitamin D and calcium dietary intake in the U.S. is far below the RDA in all female age groups, particularly for the elderly. (5) Reduction of breast cancer risk, and simultaneously osteoporosis, might be achieved by increasing dietary intake of calcium and vitamin D to RDA levels. This may be particularly applicable to females during puberty and adolescence.