To determine the effects of reversible medical castration on prostatic size and symptoms we treated 15 patients with benign prostatic hypertrophy with a long-acting GnRH analog, leuprolide (1 mg/day sc), for a minimum of 4 months. The men's serum testosterone, dihydrotestosterone, and estradiol concentrations fell to very low levels within 4-6 weeks after the initiation of treatment. Transrectal ultrasonography of the prostate demonstrated an average shrinkage of 40%after 4 months of treatment (n = 15) and 46%after 6 months of treatment (n = 11). All 15 men had improvement in urinary flow and, to a lesser extent, in nocturia and frequency. The sideeffects of the therapy were decreased potency and flushing. The most dramatic improvement occurred in 4 of the 5 men who had complete urinary obstruction before treatment. One man had a suprapubic cystotomy tube removed during the fifth treatment month. Two other men who had Foley catheters before treatment are voiding well without catheters since their third treatment month. Another man who had a very large prostate (300 g) before treatment had one successful voiding trial, although he still has a suprapubic cystotomy tube. One man decided to stop treatment after 6 months. Two months later his hormone values and prostate size had returned to pretreatment levels. One man treated during the fourth and fifth months with fluoxymesterone in addition to leuprolide had regrowth of his prostate while receiving this androgen. We conclude that leuprolide treatment of men with benign prostatic hypertrophy results in shrinkage of prostatic size and concomitant improvement in the obstructive symptoms of prostatism. The prostatic shrinkage reverses when treatment is discontinued or combined with androgen.