The recent discovery of the overexpression of P504S/alpha-methylacyl coenzyme A racemase (AMACR) in prostate cancer is a successful example of translating an advanced molecular finding into clinical practice. AMACR (P504S) has been proven to be one of the few biomarkers that can help distinguish cancer from benign cells, with high sensitivity and specificity for prostate carcinoma. It is the first gene identified by the analysis of complementary DNA microarray profiles from prostate tissue to be used as a tissue tumor marker in clinical practice and to improve the diagnosis of prostate cancer. This review focuses on the study of AMACR (P504S) expression in prostate cancer, premalignant lesions, benign prostate tissues, and other normal and malignant tissues and a discussion of its clinical usefulness. We emphasize the interpretation of the AMACR immunohistochemical results in routine surgical pathology practice and also discuss the potential future applications of this marker and the possible role of AMACR in the pathogenesis of cancer development.