Although the changes which oestrogens induce in the prostate differ in detail from species to species, in general they always begin in epithelial and fibro-muscular proliferation, and usually result in gross enlargement of the organ. This fact is primarily responsible for the belief that the clinical disorder of benign prostatic hypertrophy is also due to oestrogenic stimulation, a view which in large part is responsible for the use of androgens as a counteracting measure in the treatment of the condition. As yet, however, there is little direct information about the effects of oestrogens on the human prostate. For that reason it is of interest to report the observations which form the subject of this paper. Hamilton, Heslin & Gilbert [1937] state that no prostatic changes were induced in men who were given 13 injections of 50 to 150 μg. of oestrone over a period of 26 days, their conclusion being