Prolactin, a hormone involved in normal breast development and lactation, has been hypothesized to be important in the etiology of breast cancer. This review summarizes in vitro, animal, and epidemiologic data supporting this hypothesis. Experimental evidence indicates that prolactin can promote cell proliferation and survival, increase cell motility, and support tumor vascularization. Animal data suggest that prolactin can increase tumor growth rates and the number of metastases, as well as induce both estrogen receptor +(ER) and ER--tumors in a transgenic mouse model in which ER+ tumors are very rare. Epidemiologic data for premenopausal women are sparse; however a recent study with 235 cases reported a significant positive association between plasma prolactin levels and breast cancer risk. Studies in postmenopausal women have reported a positive association as well, and in the largest study (n=851 cases) the association was strongest for ER+ tumors. Overall, the available data support the hypothesis that prolactin increases risk of breast cancer. Future research directions include better characterizing the potential interplay between prolactin and estrogen and determining whether genetic variability in prolactin-related genes is associated with breast cancer risk.