Mammary tumors represent the most common neoplastic disease of the female dog, and the incidence in female dogs is much higher than in women. Whereas the influence of sexual steroids on breast cancer development in dogs has been studied, very little is known about the role of prolactin. New studies show that until recently, the importance of prolactin in human breast cancer development and progression has been highly underestimated. Prolactin plays a role in promoting benign as well as malignant neoplastic cell growth in breast cancer in vitro and in vivo. Sporadic publications proposed a tumor promotor role in the dog. The goal of this review is to summarize our knowledge about prolactin and human breast cancer as well as canine mammary tumorigenesis, and propose future research in this area.