Statistics indicate that the cure-rate for breast cancer has improved little over the last 35 years. That fact and the gradual increase in incidence of mammary cancer among women of the Western World serve to stimulate investigations to learn how and why mammary cancer develops. To examine the problem from a new perspective, we review the prevalence of mammary cancer reported in domesticated and zoo animals. The disease proves to be exeptionally rare in all mammals except: 1) cats and dogs, which live among humans, and 2) certain strains of laboratory mice and rats. Since the mammary tumors of these animals are sensitive to certain hormones, drugs, dietary changes etc., they serve as ideal models for studying the etiology of the disease. Because hormones promote growth and development of the mammary gland they are especially important in the pathogenesis of breast cancer. Studies of viral etiology of mammary cancer in animals are compared with those reported in humans. From the fact that a completed first pregnancy at the age of 18 protects a woman against developing a mammary cancer later in life, but a first pregnancy at about 30 years does not, some investigators believe that high levels of estriol during pregnancy act to protect the breast. From our review of breast cancer we postulat" instead that it is the apocrine function of the breast itself that affords the protection of early birth. Carcinogenic substances taken up by the estrogen-sensitized developing epithelium are discharged with the colostrum before they can transform cells. When the first birth occurs however at age 30 the carcinogens absorbed and stored in the estrogenized epithelium from puberty on have many years for transforming the cells. Dogs and cats exposed to the same polluted world as man develop mammary cancers more commonly when they first bear young late in life. Those receiving antifertility agents develop more breast cancers than those which bear first when young. Cows develop no mammary cancers because they lactate so much. High doses of carcinogens readily induce mammary tumors in some rat strains, more readily when given after a pregnancy than before. Oriental women who move to America and change their living and dietary habits develop more often mammary cancer than those who stay in their home country.