Seven patients suffering from prostatic cancer were treated with a depot form of D-Trp-6-luteinizing hormone-releasing hormone (LH-RH), a LH-RH agonist analogue (3 mg i.m. every 28 days) for a period of 24-32 months. The peptide induced a sharp and long-lasting inhibition of both gonadotropin and testosterone secretion. A sustained suppression of pituitary and testicular function was observed 40 days after the treatment was suspended. Testicular biopsies performed in all patients showed a marked impairment of Leydig cell mass and a complete spermatogenic arrest with a tubular derangement and fibrosis. Results indicate that the long-term continued gonadotropin-releasing hormone agonist analogue therapy induces not only a functional inhibition of testicular androgenesis but also anatomical testicular damage whose reversibility does not seem to be probable.