PURPOSE: Serum prostate specific antigen (PSA) is widely used for the early detection of prostate cancer, with biopsies often being performed when the serum concentration exceeds a defined threshold. The concept of prostate specific antigen density (PSAD) was introduced to enhance the specificity of serum PSA for cancer. We investigated the change in serum PSA and PSAD in patients with clinical prostatitis after antibiotic treatment. MATERIALS AND METHODS: A series of 39 patients with chronic prostatitis whose serum PSA and PSAD exceeded 4.0 ng/ml and 0.15 ng/ml/cm3, respectively, were reviewed retrospectively. After they received 4 to 8 weeks of antibiotics, the serum PSA concentration was remeasured. Men with persistently elevated serum PSA concentrations underwent prostate biopsy. RESULTS: The mean pretreatment serum PSA and PSAD was 8.73 ng/ml and 0.24 ng/ml/cm3, respectively. After antibiotic treatment, the mean values decreased significantly, to 4.55 ng/ml and 0.13 ng/ml/cm3, respectively (p<0.05). The mean decrease in serum PSA was 4.19 3.78 ng/ml, a mean relative change 44.3 31.7%. Among 39 men, 22 (56.4%) had their serum PSA and PSAD values return to the normal ranges. Prostate biopsy in 24 patients with persistently elevated serum PSA or PSAD after antibiotic therapy revealed prostate cancer in 4. CONCLUSIONS: Prostatitis is one of the most important factors in serum PSA and PSAD elevation in men without clinically detectable prostate cancer. A decrease in PSA and PSAD after antibiotic treatment can help in avoiding unnecessary prostate biopsies.