The ability to diagnose prostate carcinoma would be improved by the detection of a tumor-associated antigen. P504S, a cytoplasmic protein, was recently identified by cDNA library subtraction in conjunction with high throughput microarray screening from prostate carcinoma. The aim of this study was to establish the pattern of expression of P504S in prostate carcinoma and benign prostatic tissue. A total of 207 cases, including 137 cases of prostate carcinoma and 70 cases of benign prostate, from prostatectomies (n = 77), prostate needle biopsies (n = 112), and transurethral prostate resections (n = 18) were examined by immunocytochemistry for P504S. P504S showed strong cytoplasmic granular staining in 100% of prostate carcinomas regardless of Gleason scores and diffuse (>75% of tumor) staining in 92% of cases. In contrast, 171 of 194 (88%) of benign prostates, including 56 of 67 (84%) benign prostate cases and 115 of 127 (91%) cases of benign glands adjacent to cancers were negative for P504S. The remainders of benign prostates were focally and weakly positive for P504S. The staining pattern of these normal glands was different and easily distinguishable from that observed in prostate carcinoma. Expression of P504S was not found in basal cell hyperplasia, urothelial cells/metaplasia and small atrophic glands that may mimic prostate carcinoma. Our findings indicate that P504S is a highly sensitive and specific positive marker for prostate carcinoma.