Although vaccination is most effective when used to prevent disease, cancer vaccine development has focused predominantly on providing therapy against established growing tumors. The difficulty in developing prophylactic cancer vaccines is primarily due to the fact that tumor antigens are variations of self proteins and would probably mediate profound autoimmune complications if used in a preventive vaccine setting. Here we use several mouse breast cancer models to define a prototypic strategy for prophylactic cancer vaccination. We selected alpha-lactalbumin as our target vaccine autoantigen because it is a breast-specific differentiation protein expressed in high amounts in the majority of human breast carcinomas and in mammary epithelial cells only during lactation. We found that immunoreactivity against alpha-lactalbumin provides substantial protection and therapy against growth of autochthonous tumors in transgenic mouse models of breast cancer and against 4T1 transplantable breast tumors in BALB/c mice. Because alpha-lactalbumin is conditionally expressed only during lactation, vaccination-induced prophylaxis occurs without any detectable inflammation in normal nonlactating breast tissue. Thus, alpha-lactalbumin vaccination may provide safe and effective protection against the development of breast cancer for women in their post-child-bearing, premenopausal years, when lactation is readily avoidable and risk for developing breast cancer is high.