Testicular responsiveness to 5,000 IU of human chorionic gonadotropin was evaluated in 14 patients with prostate cancer who were being treated with a slow-release luteinizing hormone-releasing hormone agonist for a median of 21 months. Serum testosterone response to human chorionic gonadotropin was markedly reduced in most patients, with the median level increasing from 0.25 to 1.65 nmol. per l. A second human chorionic gonadotropin test was repeated later in 5 patients who had been off treatment for a median of 6 months. Median serum testosterone levels increased to a maximum of 2.6 nmol. per l. compared to 28.2 nmol. per l. in an age-matched control group (p equals 0.008). Therefore, we conclude that long-term treatment with luteinizing hormone-releasing hormone agonists in elderly men leads to gonadal impairment that may not be as reversible as generally suggested.