Prostate cancer in most European countries is the second most frequent cancer in males and the second most frequent cause of cancer death. Prostate specific antigen (PSA) is an important tumour marker, which relates to many aspects of this disease. It has been shown that PSA is helpful in the early diagnosis of prostate cancer and in this respect is superior to other available tests like rectal examination and transrectal ultrasonography. PSA is also helpful in staging of locally confined disease. It can be used to identify or exclude local extension of disease, if combined with T category and grade of differentiation determined on biopsy. The same parameters also give an indication of the presence of lymph node metastases, which may prevent unnecessary and invasive staging procedures in certain groups of patients with favourable prognostic factors and a low PSA value. PSA is less suitable as a marker for metastatic disease. Progression of untreated prostate cancer in various stages can be monitored by PSA. The true value of the marker in this respect is still underexplored. It may be possible that PSA will be shown to differentiate effectively between aggressive and non-progressive disease. In this respect, it could become an essential tool to identify those patients that may not require treatment at all. PSA is also a useful marker for therapy response. An elevation of PSA after radical prostatectomy indicates local or metastatic progression, which will occur within 1-2 years. PSA is an androgen dependent enzyme and decreases under endocrine treatment. It is unexplained why in spite of its endocrine dependent character, PSA rises with endocrine independent progression of prostate cancer.