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Establishing a definitive diagnosis of malignancy in prostate needle biopsies with very small foci of adenocarcinoma is a major diagnostic challenge for surgical pathologists. A positive diagnostic marker specific for prostatic adenocarcinoma may enhance our ability to detect limited prostate cancer and reduce errors in diagnosis. P504S, also known as alpha-methylacyl-CoA racemase, recently identified by cDNA subtraction and microarray technology, might serve as such a specific marker because it has been demonstrated to be highly expressed in prostatic adenocarcinoma, but not in benign prostatic glands. However, whether small foci of carcinoma can be reliably detected by this marker is a crucial question for its clinical application. The aim of this study was to assess the utility of P504S immunohistochemistry in detecting small amounts of prostate cancer in prostate needle biopsies. A total of 142 prostate needle biopsies, including 73 cases with a small focus of prostatic adenocarcinoma (=1 mm) and 69 benign prostates, were examined by using immunohistochemistry for P504S and high molecular weight cytokeratin (34betaE12). P504S immunoreactivity was found in 69 of 73 cases (94.5%) of carcinoma but not in any benign prostates (0 of 69) or benign glands adjacent to malignant glands. The 34betaE12 immunostaining confirmed the absence of basal cells in the focus of carcinoma in all 73 cases. The high specificity and sensitivity of P504S in the detection of minimal prostatic adenocarcinoma indicated its potential diagnostic value in clinical practice. Using a combination of P504S and 34betaE12 can help the diagnosis of limited prostatic adenocarcinoma on needle biopsy.