To determine the effects of reversible medical castration on prostatic size in patients with benign prostatic hypertrophy (BPH), 3 patients with BPH were treated with a GnRH analogue, leuprolide, for six months at a dosage of .2ml (1 mg) s.c. daily. Serum testosterone, dihydrotestosterone and estradiol fell to castration levels 4-6 weeks after the initiation of treatment and remained low throughout the study period. Transrectal ultrasonography of the prostate demonstrated an average decrease in prostatic volume of 58% at 6 months, with the greatest rate of decrease occurring during the 2nd to 5th months of treatment. One man who had acute urinary retention before treatment was subsequently able to void extremely well. In a second man the symptoms of prostatism diminished but in the third urinary frequency and nocturia persisted in spite of a reduction in prostatic size, presumably because his symptoms were due to renal insufficiency.