Prostate-specific antigen (PSA) is the most important of all tumor markers in that it has significant applications in all aspects of the management of men with prostatic disease. Certainly, the most important utilization of PSA is for the early detection of this most ubiquitous of all human neoplasms. This article reviews the salient features of PSA, with particular emphasis on strategies to improve its utility in the diagnosis of prostate cancer. So-called PSA derivatives--including age-specific PSA, PSA velocity, and PSA density--are discussed. With the recognition of molecular forms of PSA, however, the ratio of free-to-total PSA, and now the complex form of PSA, have been shown to be more specific indicators of the presence of malignancy. Significant public interest and research efforts in prostate cancer have resulted in numerous advances over the past decade. The discovery of PSA and the development of assays to measure it will undoubtedly be recorded as one of the most important advances in the management of men with prostate cancer.