Because mouse mammary tumor virus (MMTV; the Bittner virus) is the proven cause of breast cancer in both field and experimental mice, similar viruses have long been suspects as a potential cause of human breast cancer. MMTV-like viral genetic material has been identified in human breast tumors, but there is no definitive evidence whether MMTV is causal and not merely an innocuous infection in humans. High-risk human papilloma viruses (HPVs), Epstein-Barr (EBV), and other viruses also have been identified in human breast tumors, but again there is no definitive evidenc e for a causal role. Any viral hypothesis as a cause of breast cancer must take into account the most striking epidemiologic feature of human breast cancer, the three- to sixfold differences in mortality and up to eightfold differences in incidence between some Asian and Western populations. These differences dramatically lessen to a two- to threefold difference within one or two generations of migration of females from low to high risk of breast cancer countries. In this chapter, a plausible explanation for these phenomena is offered; that is, the hypothesis that oncogenic viruses such as MMTV and high-risk HPVs may initiate some breast cancers in most populations. Furthermore, dietary patterns are suggested to determine circulating sex hormone levels, which in turn promote the replication of the hormone-dependent viruses MMTV and HPV. In addition, diet and hormones promote growth of both normal and malignant cells. Finally, the hypothesis that migrants from low to high risk of breast cancer countries change their food consumption patterns is suggested, which leads to higher circulating hormone levels, which in turn promotes viral replication, which initiates breast oncogenesis, which is enhanced by sex and growth hormones.