Prostate Specific Antigen (PSA) is the most accurate serum marker for cancer of the prostate. However, sensitivity and specificity are suboptimal, especially at the intermediate levels between 4.1 and 10.0 ng/ml (monoclonal). For intermediate PSA levels, prostate specific antigen density (PSAD) provides unique information regarding the need for biopsy and the likelihood of prostate cancer. This article summarizes the utility of PSAD in diagnosing and treating prostate cancer.