The effect of medical oophorectomy induced by treatment with the luteinizing hormone-releasing hormone (LH-RH) agonist [D-Trp6,des-Gly-NH2(10)]LH-RH ethylamide was studied in 34 patients with laparoscopically proven endometriosis. Tamoxifen was administered during the 1st month of therapy to prevent flare-up of the disease during the estrogen surge. Fifteen women had a decrease of their laparoscopy scores translated into an improvement in the stage of disease, whereas in 12 others, the decrease in their scores was not enough to allow a change of disease stage. The 2nd laparoscopy was not performed in 7 women. Medical oophorectomy, after daily injection of the LH-RH agonist (LH-RH-a), was accompanied by low levels of circulating estradiol. The serum concentration of all delta 4-3-ketosteroids was significantly decreased during medical oophorectomy, whereas the level of circulating delta 5-3 beta-hydroxysteroids was not altered except for pregnenolone. The present data indicate that medical oophorectomy induced by an LH-RH-a in association with tamoxifen is a very efficient and well tolerated therapy in endometriosis.