The local endocrine environment of the breast may have stronger relations to breast cancer risk than systemic hormones. Nipple aspiration fluid (NAF) provides a window into this milieu. We hypothesized that the correlations between proteins and steroid hormones in NAF are stronger, and specific relationships may reveal links to breast cancer risk. NAF and blood samples were obtained simultaneously from 54 healthy women and from the contralateral unaffected breast of 60 breast cancer patients. The abundance of five proteins, superoxide dismutase (SOD1), C-reactive protein (CRP), chitinase-3-like protein 1 (YKL40), cathepsin D (CatD), and basic fibroblast growth factor (bFGF) in NAF was measured using ELISA. The NAF and serum concentrations of estradiol, estrone, progesterone, androstenedione, testosterone, and dehydroepiandrostrerone (DHEA) were measured using ELISA or RIA. The correlations between proteins and hormones revealed that NAF proteins correlated with each other: SOD1 with CRP (R = 0.276, P = 0.033) and CatD (R = 0.340, P = 0.0036), and bFGF with CRP (R = 0.343, P = 0.0021). NAF proteins displayed significant correlations with NAF steroids, but not with serum steroids: SOD1 with DHEA (R = 0.333, P = 0.019), YKL40 with testosterone (R = 0.389, P = 0.0012), and bFGF negatively correlated with testosterone (R = -0.339, P = 0.015). The regulation of YKL40 and bFGF by testosterone was confirmed in breast cancer cell lines. In summary, NAF proteins were more strongly related to local hormone levels than to systematic hormone levels. Some proteins were specifically correlated with different NAF steroids, suggesting that these steroids may contribute to breast cancer risk through different mechanisms.