Zinc α-2-glycoprotein (ZAG) is a M r 41, 000 glycoprotein secreted by a variety of normal epithelia. ZAG was recently shown to stimulate lipolysis in adipocytes, leading to the development of cachexia in animals with ZAG-producing tumors. To understand the possible contribution of ZAG to the development of cachexia in men with prostate cancer, ZAG production by normal and malignant prostate tissue was investigated using immunohistochemical assays. Anti-ZAG monoclonal antibodies reacted strongly with normal prostate epithelium but not with other components of prostate or seminal vesicles. The majority of prostate cancers tested (35 of 48; 73%) also reacted with anti-ZAG antibodies. High-grade tumors expressed significantly less ZAG than moderate-grade tumors (mean ZAG score 1.1 versus 1.9; P < 0.01). Men with ZAG-producing prostate carcinomas had elevated levels of serum ZAG relative to their normal age- and race-matched controls (P < 0.02). Furthermore, s.c. growth of human ZAG-producing murine tumors in syngeneic mice and orthotopic growth of ZAG-producing human prostate carcinomas in nude rats resulted in readily detectable levels of human ZAG in the serum. Taken together, these studies show that ZAG production by prostate cancer can lead to systemically elevated serum ZAG levels that may be useful diagnostically. The effects of elevated systemic ZAG on cachexia-associated complications in patients with advanced prostate cancer deserves additional investigation.