PURPOSE: This controlled prospective study aims to investigate the possible effects of antibiotic treatment on prostate-specific antigen (PSA) and its derivatives, and consequently on the transrectal biopsy rates, in the diagnosis of prostate cancer. PATIENTS AND METHODS: One hundred and forty patients aged 45 to 70 years old, with a PSA level between 2.5 and 10 ng/mL and normal digital rectal examinations (DRE), were included in this study between June 2009 and November 2010. The patients were randomly assigned into two groups. The first group received oral levofloxacin 500 mg 1*1 for 21 days; the second, the control group, was given no treatment. Initially, total PSA, free PSA, a DRE, urinary ultrasonography (including prostate volume, postvoiding residual urine), uroflowmetry, International Prostate Symptom Score, National Institutes of Health Chronic Prostatitis Symptom Index, and International Index of Erectile Function tests were performed. All of these were repeated at the end of 3 weeks of antibiotic treatment. An additional PSA measurement was also performed at day 10 of the treatment. All patients underwent transrectal ultrasonography (TRUS) guided prostate biopsy at day 21, just the day after the final (third) PSA sampling. RESULTS: The mean age of the patients was 59.6 years. Overall, in 23 patients, prostate cancer was detected, including those found in the rebiopsies. Statistically, there were significant changes in values of PSA and its derivatives in the treatment group (from 5.31 to 4.69 and 4.58 ng/mL, consecutively). Focusing on prostate cancer patients in both the treatment and control groups, however, we did not detect any significant change in the same parameters. CONCLUSION: Antibiotic treatment given to the patients with a PSA level between 2.5 and 10 ng/mL can be beneficial, before a decision for TRUS guided prostate biopsy, just in a limited subgroup, by reducing the PSA levels below the threshold value. Considering the large population of patients in the gray zone, however, it still does not provide clear solid evidence for avoiding unnecessary prostate biopsies.