The bovine leukemia virus (BLV) is an oncogenic retrovirus which belongs to the Retroviridae family and is known for its worldwide distribution. It was discovered in 1969, producing an infection called enzootic bovine leucosis (EBL) which is one of the most common neoplastic diseases in cattle. Recently some investigations have shown that this virus could be associated with human breast cancer, which implies that BLV could be an agent of a zoonosis, with the ability to also infect humans. In order to demonstrate this hypothesis, 56 cases diagnosed as canicular carcinoma in the “Hospital Universitario San Ignacio” in Bogota, Colombia, during the year 2004 were chosen for analysis. Immunohistochemistry (IHC) was used on routinely formalin-fixed, paraffin embedded tissue samples for the detection of the BLV glycoprotein gp51 in the breast cancer tumor cells. The results show 7% positive samples, indicating that the virus was present in humans as well as confirming in vitro studies which show the susceptibility of human cells to BLV infection. These results allow us to suggest that BLV is present as an active provirus capable of producing the proteins that its progeny virus particles will use to leave its host cell. At the same time patient variables such as: place of origin and of birth, previous history of cancer in the patient and his family, and occupation were evaluated in relation to the presence of the virus; but no statistical relationship was demonstrated.