The name prostate-specific antigen has been given to a protein that now is known not to be prostate-specific; however, prostatic tissue does produces extremely high levels of PSA and secrets it into the seminal plasma. Seminal plasma contains about 1 million micrograms/L of PSA and is the richest source of PSA reported. The biologic fluid with the second highest PSA concentration, however, is nipple aspirate fluid from the female breast (up to about 5000 micrograms/L), and the third is milk from lactating women (up to 300 micrograms/L). Male serum PSA is usually less than 4 micrograms/L. In nonprostatic tissues, PSA exists mainly in its free molecular form, but PSA-ACT complex is also present in most of the fluids that contain PSA, such as breast secretions and amniotic fluid. The gene expression and protein production of PSA in nonprostatic tissues are under the regulation of steroid hormones via their receptors. Androgens, glucocorticoids, and progestins up-regulate the PSA gene expression, resulting in an increase of protein production. Estrogen by itself seems to have no effect on PSA regulation, but it can impair PSA production induced by androgen. It remains unknown whether PSA is enzymatically active and what is the physiologic role of PSA in nonprostatic tissues. It is speculated that PSA may be involved in the regulation of growth factors. Measuring PSA in breast cancer cytosol, breast-nipple aspirate fluid, and female serum may have potential clinical utilities, including breast cancer prognosis, breast cancer risk assessment, and evaluation of androgen excess. Further studies are needed to identify the exact function and regulation of PSA in nonprostatic tissues and to explore the clinical application of this protein.