A case-control study was conducted in order to test the hypothesis that a reduced exposure to human seminal factors in the early reproductive life of women is a risk factor in breast cancer. The relative risk of exposure to the hypothetical semen-factor deficiency is 4.7 times greater for breast-cancer patients than for the controls. When the contraceptive methods alone are taken into account, the relative risk increases up to 5.3. About 16 percent of the women using barrier methods (condom, and other) and 3.4 percent of women using non-barrier contraceptive methods (diaphragm, pill, IUDs, rhythm, tubal ligation, and other) would develop breast cancer. The risk of developing breast cancer within the same population in the U.S. is 5 times greater in women who use barrier methods than in women who use non-barrier contraceptive methods. The reduction in the incidence of breast cancer by eliminating the barrier contraceptive techniques would be not less than 50 percent in married women in the entire population. The possibility of preventive measures within an adequate family planning program is emphasized.