In his book, "What Your Doctor May Not Tell You About Prostate Health and Natural Hormone Supplementation", Dr. John Lee writes that in the prostate (and in the hair follicles) is an enzyme (5-alpha-reductase) that converts testosterone into dihydrotestosterone (DHT). Higher DHT levels in hair follicles is a primary cause of male pattern baldness. He states that DHT stimulates proliferation of prostate cells enlarging the prostate gland and narrowing the urethral channel, leading to urination problems, and speculation that elevated DHT is the cause of prostate cancer.
Since progesterone is a potent inhibitor of 5-alpha-reductase, the decline of progesterone in aging males plays a role in increasing the conversion rate of testosterone to DHT. Supplementing with progesterone cream helps restore normal inhibition of 5-alpha-reductase, thus preventing testosterone from changing into dihydrotestosterone (DHT), which stimulates proliferation of prostate cells. Dr. Lee suggests that men over the age of 40 use 8 - 12 mg of
natural progesterone applied directly to their scrotum twice daily every day of the week to protect against prostate cancer.
Not sure how accurate all of these studies are, but:
At the Institute for Biological Medical Experiments in Buenos Aires it was shown progesterone shrank prostate weight in test animals as well as reduced 5-alpha reductase activity.
At Central Hospital University in Paris progesterone was shown to inhibit the formation of DHT as well as binding of it to the prostate. DHT content in the prostate is the single most causative factor in prostate disease.
At the Biochemical Medical Laboratory in France the doctors demonstrated in human BPH tissue there are more progesterone receptors which show how responsive the prostate is to this hormone.
At the Institute of Clinical Chemistry in Bochum, Germany progesterone in human BPH tissue reduced the activity of 5-alpha reductase strongly.