Prostate-specific membrane antigen (PSMA) serum levels have been proposed to be of prognostic significance in patients with advanced prostate disease. The objective of the present study was to confirm PSMA serum expression by Western blot techniques, to determine whether such data could assist in the differentiation of benign from malignant prostatic disease, and to determine the suitability of serum PSMA measurements in predicting recurrent or progressive prostate malignancies. We measured PSMA, a transmembrane glycoprotein identified in prostate epithelial cells, in the sera of 236 normal individuals and cancer patients by Western blot analysis. Within the normal male population, PSMA levels increase with age and were found to be significantly elevated in subjects more than 50 years of age when compared to those of younger men. We did not confirm previous reports that serum PSMA measurements could distinguish late-stage prostate carcinoma from early-stage prostate carcinoma, nor did we find PSMA to be more effective than prostate-specific antigen in monitoring prostate cancer patient prognosis. Furthermore, we found elevated serum PSMA in healthy females, and, similar to the healthy male population, the levels increased with age, with the highest levels found in the sera from breast cancer patients. These latter observations further support that PSMA is not a specific biomarker for prostate cancer and that a variety of normal and diseased tissue may contribute to the serum levels of PSMA.