Objectives: Prostate cancer is an interesting tumor for endocrine investigation. The prostate-specific antigen/free testosterone (PSA/FT) ratio has been shown to be effective in clustering patients in prognostic groups as follows: low risk (PSA/FT ≤0.20), intermediate risk (PSA/FT >0.20 and ≤0.40) and high risk (PSA/FT >0.40 and ≤1.5). In the present study we explored the total PSA and FT distributions, and linear regression of FT predicting PSA in the different groups (PSA/FT, pT and pG) and subgroups (pT and pG) of patients according to the prognostic PSA/FT ratio. Patients and Methods: The study included 128 operated prostate cancer patients. Pretreatment simultaneous serum samples were obtained for measuring free testosterone (FT) and total PSA levels. Patients were grouped according to the total PSA/FT ratio prognostic clusters (≤0.20, >0.20 and ≤0.40, >0.40), pT (2, 3a and 3b+4) and pathological Gleason score (pG) (≤6, = 7 >3 + 4, ≧7 >4 + 3). The pT and pG sets were subgrouped according to the prognostic PSA/FT ratio. Linear regression analysis of FT predicting total PSA was computed according to the different PSA/FT prognostic clusters for the: (1) total sample population, (2) pT and pG groups, (3) intraprostatic (pT2) and extraprostatic disease (pT3a/3b/4), and (4) low-intermediate grade (pG ≤6) and high-grade (pG ≧7) prostate cancer. Results: Analysis of variance always showed highly significant different PSA distributions for (1) the different PSA/FT, pT and pG groups; and (2) the pT and pG prognostic subgroups. Significant FT distributions were detected for the (1) PSA/FT and pT groups; and (2) the pT2, pT3a and pG ≤6 prognostic PSA/FT subgroups. Correlation, variance and linear regression analysis of FT predicting total PSA was significant for (1) the PSA/FT prognostic clusters, (2) all the pT2 and pT3a subgroups, and (3) the pT3b/4 subgroup with PSA/FT >0.20 and ≤0.40, and (4) all the pG subsets. Linear regression analysis showed that the slopes of the predicting variable (FT) were always highly significant for patients with (1) intraprostate and extraprostate disease, and (2) low-grade and high-grade prostate cancer. Conclusions: According to the prognostic PSA/FT ratio, significantly lower levels of FT are detected in prostate cancer patients with extensive and high-grade disease. Also, significant linear correlations of FT predicting PSA are assessed in the different groups and subgroups of patients clustered according to the prognostic PSA/FT ratio. Confirmatory studies are needed.