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. The risk of developing breast cancer within the same population in the U.S.A. is estimated as 5 times greater for women who use barrier methods (condom and other) than for women who use non-barrier contraceptive methods (diaphragm and other). The reduction of the incidence of breast cancer by eliminating the barrier contraceptive techniques would be not less than 50% in married women in the population.