Purpose: The objectives of this study were to provide data about the intra-individual biological variations in prostate specific antigen levels for young men and to evaluate the significance of changes in serial results from an individual. Materials and Methods: The study group comprised 26 healthy male volunteers; median (range) age was 32.5 (22-49) years. Four blood samples were obtained from each subject, one at each 14-day interval. Serum PSA levels were measured by a chemiluminescent microparticle immunoassay (CMIA) on a random-access analyzer. Results: The intra-individual biological variation for total serum prostate specific antigen was 20.7%. The reference change value (RCV) for tPSA was 49.4% (p <0.05) Conclusion: The estimates of intra-individual biological variation and RCV for total PSA in the group were similar to the published mean data on elderly subjects. For men under 50 years old, a change greater than 50% in tPSA levels in a 14-day interval indicates a significant change. The biological variation should be considered when evaluating PSA results for all age groups. Moreover, for the reliable use of the PSA assay in clinical practice determinations of CVA, CVI, and RCV are required. When screening and monitoring, especially familial forms of prostate cancer in younger men, our estimates of biological variation and RCV for total PSA may be considered.