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Prostate specific antigen (PSA), which has high organ specificity, is an excellent tumor marker that has played a significant role in the diagnosis and treatment of prostate cancer. Screening for prostate cancer using PSA is now widely employed in Japan, and increased detection of cases with organ-confined prostate cancer is hoped to result in a decreased number of cancer-specific deaths. Although PSA has also played a critical role in as a marker for staging, assessment of treatment, and recurrence of prostate cancer, many useless biopsies are performed due to its low cancer specificity. To increase the specificity in prostate cancer detection, PSA-related markers including PSAD (PSAPZD), PSAV, and age-specific PSA were advocated, and the ratio of free PSA to total PSA (% free PSA) is also used in a clinical setting. However, since those markers can not satisfactorily exclude benign prostate diseases, various molecular forms of PSA have been analyzed using proteomics and glycomics. Recently, it was demonstrated that plasma free PSA consisted of precursor PSA (pPSA) and other isoforms, suggesting that [-2] pPSA may be a helpful marker for prostate cancer. Our group reported that a sugar chain structure of PSA in the serum of prostate cancer patients is different from that of patients with benign prostate hyperplasia. The different sugar chain structure of PSA can be easily detected by a conventional method and is expected to be useful for differential diagnosis between malignant and benign prostate diseases.