The histologic composition of the breast is reflected mammographically by density and parenchymal pattern. Research suggests that the parenchymal pattern is a result of exposure to past and current hormones as well as reproductive events which influence susceptibility to breast cancer. The variation observed in mammographic density may be a result of the effect of hormones and/or growth factors. One possible hormone is prolactin, which acts directly on mammary epithelium to produce lobuloalveolar development. Prolactin acts as a mitogen and differentiating agent, especially during pregnancy and lactation. The specific objective of this application is to correlate serum prolactin levels and breast density in postmenopausal women using a cross-sectional study design. I also want to examine the genetic variability in the prolactin gene and prolactin gene receptor in order to understand how they may influence breast density. To date, no study has examined the association between prolactin and breast density. Nor has any study examined the association of genetic polymorphisms in the prolactin and its receptor genes and breast density. This training will form the basis of my Ph.D. work in cancer epidemiology and will provide the foundation for my research career.