A total of 4 men with benign prostatic hypertrophy who underwent medical castration therapy with a long-acting gonadotropin-releasing hormone agonist (leuprolide) for more than 6 months elected to add an estrogen transdermal patch (0.05 mg. to the skin biweekly) to the leuprolide regimen. The average prostatic size (transrectal ultrasound), serum prostate specific antigen (PSA) levels and symptoms of prostatism were dramatically decreased with leuprolide alone. The addition of estrogen for 6 months did not result in any change in prostate size, symptoms or serum PSA levels over that seen with leuprolide alone. The development of squamous metaplasia was noted in 1 man with leuprolide alone and in 1 man after the addition of estrogen. Immunohistochemical staining with anticytokeratin 903 antibodies reveals that squamous metaplasia appears to arise from prostatic basal cells. We postulate that the target cell for estrogen action in the prostate is the prostatic basal cell. In the absence of androgen the only direct effect of estrogens is the induction of squamous metaplasia.