Purpose: The greatest concern in the androgen replacement therapy (ART) is the possible side effects to the prostate. We evaluated the effects of ART focusing on the prostate specific antigen (PSA). Materials and Methods: From 2003 to 2006, 47 patients 44 to 75 years old (mean age 60.1) received ART. At baseline and after ART, digital rectal examination, serum testosterone and PSA measurement and transrectal ultrasonography were evaluated. Mean follow-up was 7.9 months (range 1 to 41). Patients were classified into two groups based on the initial PSA level, as PSA levels of 2.5 ng/ml or greater (group 1) (n=29) and PSA levels of less than 2.5 ng/ml (group 2) (n=18). Results: ART significantly increased serum testosterone, PSA and free PSA levels. However, prostate volume did not change significantly. When serum PSA was compared, the increase of PSA levels was greater than in high PSA group (group 1) than in group 2, although the ratio between the two groups in PSA increase was 38.3% for group 2 and 18.2% for group 1, respectively. A total of 4 patients (16.7% of group 1 and 3.4% of group 2) with a serum PSA level greater than 4 ng/ml after ART underwent a prostate biopsy but no patients were found to have prostate carcinoma. Conclusions: Rates of PSA elevation (>4 ng/ml) and prostate biopsies were higher in patients with high baseline PSA level (> or =2.5 ng/ml) than in those with PSA level less than 2.5 ng/ml who received ART. However, our findings suggest that an increased risk of prostate cancer was not associated with ART.