Despite the fact that breast cancer is the most common non-cutaneous cancer and a leading cause of cancer deaths in women, accepted markers of breast cancer risk miss up to 40% of these tumors. Moreover, screening methods involving the analysis of tissue or cells are limited by the need for a surgical biopsy. Nipple aspiration is a quick, efficient, noninvasive method to obtain breast epithelial cells, the cells at risk for transformation to carcinoma. Prostate-specific antigen (PSA), a protein thought to be specific to the prostate but recently found in a subset of breast tumors, has been correlated with improved survival. The purpose of this study was to measure PSA in a group of women with increasing breast cancer risk (no risk or family history of breast cancer, precancerous mastopathy, and invasive cancer) and determine if PSA correlates with risk. Nipple aspirate fluid was obtained from the intact breast and from surgical specimens using a modified breast pump. PSA was then measured in the fluid using a highly sensitive and specific immunofluorometric procedure. PSA was found at levels ranging from 0-13,423 ng/g of total protein, and there was a significant relationship between PSA level and breast cancer risk (P = 0.001). That is, all women with no risk factors and 90% of those with a family history had high PSA levels, whereas 68% of subjects with precancerous mastopathy or invasive cancer had low PSA levels. PSA was higher in premenopausal subjects (P = 0.002). After adjusting for the effect of menopausal status, there remained a significant association between PSA and breast cancer risk. These findings suggest that PSA in nipple aspirate fluid may be a useful marker of breast cancer risk.