The prostate specific antigen (PSA) is the best marker of the prostate cancer today although not very specific of this pathology. The dynamic interpretation of this marker always has to prevail over that of overtaking a threshold. After radiotherapy, PSA can decrease after a mean interval of one to two years to a value less than 1 microg/L (predictive of recurrence-free survival). Biochemical recurrence after radiotherapy is defined by an increase of PSA by 2 microg/L or more above the PSA nadir, whether or not it is associated with endocrine therapy. The time of appearance of the recurrence and the PSA doubling time after total radiotherapy have a diagnostic value on the nature of the site of recurrence, local or metastatic.