Testosterone has a distinct role in benign and malignant diseases of the prostate. Therefore, knowledge about the physiological interactions between testosterone and the prostate and the special circumstances under testosterone substitution are of great impact for urologists.PSA value and prostate volume do not show significant changes under testosterone substitution therapy. Even if there are no long-term studies in men under substitution due to decreased testosterone, the therapy seems to be safe under regular control of the prostate with PSA and sonography, and the risk for prostate carcinoma is not increased. In hypogonadal men with high-grade PIN under testosterone substitution, 1 in 20 cases with suspicious rectal examination exhibited a carcinoma; the PSA values did not show a difference between men with or without PIN.Nevertheless, it remains unclear whether men after successful radical prostatectomy should receive testosterone substitution.