Prostate cancer has a varied natural history. Men with similar serum prostate-specific antigen (PSA) levels, clinical stages, and histologic features in their tissue specimens can have markedly different outcomes. While prostate cancer is lethal in some patients, most men die with cancer rather than because of it. Moreover, histologically apparent cancer can be found in the prostate glands of approximately 42% of men over 50 years of age who die from other causes, but the lifetime risk that a man in the US will be diagnosed with prostate cancer is estimated to be 11% and the risk of dying from the disease is only 3.1%. Consequently, appropriate disease management requires risk assessment. How likely is it that a given man's cancer will progress or metastasize over his remaining lifetime? What is the probability of successful treatment? What are the risks of adverse effects and complications of each treatment? Physicians use a variety of clinical and pathologic parameters to assess the risk that a given cancer poses to an individual patient. In addition to the accepted parameters of serum PSA level, clinical staging, and pathologic grading and staging, PSA doubling time has emerged as an important factor in the evaluation of men with newly diagnosed prostate cancer or prostate cancer that recurs after treatment. PSA doubling time can also be used as a surrogate marker for prostate cancer-specific death. This review summarizes current knowledge regarding the role of PSA doubling time as a prognostic marker in men with prostate cancer.