Predicting the prognosis of patients with prostate cancer is a clinically important problem. Previous studies have indicated that the expression of haptoglobin-related protein epitopes in samples of breast cancer in early stages was associated with earlier relapses and higher risk for tumor recurrence. Oncogenic antigen 519 (OA-519) is the new marker designation for molecules expressing haptoglobin-related protein epitopes. The objective of this immunohistochemical study was to examine OA-519 expression in prostate cancer samples and its relationship to the established prognostic indicators of tumor grade, tumor volume, and clinical stage. Forty-two consecutive tissue samples of prostate adenocarcinoma were examined using an affinity-purified anti-OA-519 antibody. Twenty specimens (48%) tested positive, whereas 22 (52%) tested negative. No staining was observed in normal or hyperplastic prostate tissue. Staining occurred in 6 of 9 (67%) grade III, 14 of 23 (61%) grade II, and in none of 10 (0%) grade I cases (I vs. II and/or III: Fisher exact test, P less than 0.006). Twenty-three of the 42 samples were transurethral resection specimens with cancer; 11 (48%) of these tested positive. The mean percentage of tissue chips with tumor, a measure of tumor volume, was significantly higher in the positive group (57%) than in the negative group (15%) (P = 0.004). The proportion of positively stained cases increased with advancing clinical stage, with 25% of Stage A cases expressing OA-519, and 46%, 67%, and 64% of Stages B, C, and D, respectively, expressing OA-519. OA-519 expression correlates with higher tumor grades, larger tumors, and possibly with advanced stage, and thus, it is potentially of prognostic value in prostate cancer.