Sixteen patients with prostatic carcinoma were treated with 200 mg of Cyproterone acetate daily. No other kind of hormonal treatment was given. Transrectal biopsies of the prostate were taken before the treatment was started, and at regular intervals afterwards. The treatment period lasted from 3 to 16 months, with an average of 9 months. A thorough examination of multiple sections from all specimens revealed no convincing signs of cellular involution. The study has demonstrated no specific or significant atrophic changes following Cyproterone acetate therapy. Some possible explanations regarding the effect of Cyproterone acetate on human malignant prostatic tissue are discussed.