The cynomolgus monkey was selected as an experimental model to investigate the role of estrogens in the pathogenesis of benign prostatic hyperplasia (BPH) because its prostate seems to be more like the human prostate than that of other primate species. The treatment of intact, adult animals with the aromatizable substrate androstenedione for 3 months resulted in no significant changes in prostate weight, but in microscopically clearly detectable estrogen-related hyperplastic changes, particularly in a marked smooth muscle activation. These effects were antagonized by simultaneous, subcutaneous treatment with the aromatase inhibitor 1-methyl-androsta-1,4-diene-3,17-dione (1-Methyl-ADD) and only partially reversed by oral treatment. The serum estradiol (E2) concentration, which was not significantly elevated after treatment with androstenedione in comparison to the control, was drastically decreased after subcutaneous treatment with 1-Methyl-ADD and moderately decreased after oral treatment. In conclusion, these results as well as the great anatomical and histological similarities between the prostate of the cynomolgus monkey and that of man indicate that it might be a suitable and interesting model for future BPH studies.