To study the effect of transrectal ultrasonography (TRUS) on serum total prostate-specific antigen (PSA), free PSA and free/total (f/t) PSA ratio and to determine the reliability of serum total and free PSA levels and f/t PSA ratio after such interventions, serum free and total PSA levels and f/t PSA ratios were determined before, immediately after and 24 h after TRUS in 44 patients who were referred to our department by their general practitioner for prostate screening. The duration of TRUS procedure and prostate volume and age were evaluated to determine whether the serum PSA levels following TRUS had changed. The total PSA and free PSA levels and f/t PSA ratio did not show any significant rise immediately after DRE and TRUS, The duration of the TRUS procedure did not appear to affect the PSA values. Age and prostate volume were not correlated with the PSA levels. Our findings revealed no significant changes in f/t PSA ratio and serum free and total PSA level, suggesting that serum free and total PSA levels and f/t PSA ratio may be more reliable parameters in the early period after TRUS, Copyright (C) 2000 S. Karger AG,Basel.