Within a few weeks, if therapy of the hormone sensitive, advanced prostate cancer (PCa) is sufficient, there will be a PSA-decrease to testify to the regression of the PCa. Free-PSA (f-PSA) is used for the differential diagnosis of the PCa. Values under 25% f-PSA in proportion to complete-PSA show the possibility of the existence of a Pca. The aim of the work was to study the behavior of f-PSA under hormonal ablation. Initial PSA and f-PSA was examined (RIA) in 76 patients (average age = 72.8 yrs. old) with advanced PCa. (metastases) proven by bone scintigraphy and/or computed tomography. During hormonal therapy (LHRH-agonists) monthly PSA and f-PSA abundance were examined. The percent amount of f-PSA was calculated and documented for at least 6 months. The initial PSA-values were 43.6 +/- 17.3, the f-PSA were 13.4% +/- 8.9. Under antihormonal therapy PSA decreased (while f-PSA increased) and after a period of 1 month the values were 27.4 +/- 14.9 (17.4% +/- 12.3), after 3 months 18.1 +/- 11.3 (24.5% +/- 9.9), after 6 months 7.9 +/- 6.8 (26.1 +/- 10.6). During the 6 months of hormonal ablation PSA-values continuously declined, while, after the first 3 months, f-PSA-values showed a behavior similar to benign hyperplasia of prostate. The therapeutic efficiency of the antihormonal therapy is clearly shown through f-PSA and PSA. There are no timely advantages between the two markers. Further investigations will show whether hormone insensitive PCa can be recognized quicker through f-PSA than from an increase in PSA.