In a multicenter study, the efficacy of and tolerance of 6 months' intranasal gonadotropin-releasing hormone agonist (buserelin) treatment (300 micrograms x 3/day) on laparoscopically verified endometriosis was evaluated in 25 patients. At second-look laparoscopy at the end of medication, the mean endometriosis score had fallen by 82.2%. All endometriosis-associated symptoms and physical findings decreased or almost disappeared during buserelin administration. After discontinuing therapy, they showed a tendency to reappear, but nevertheless they were milder after one year of follow-up, than before treatment. Seven (54%) of the 13 women wishing pregnancy actually conceived. Vaginal irregular spotting bleedings during the first 2 months occurred in 7 patients. No patient withdrew from the trial because of side effects, although almost all women developed symptoms of estrogen deficiency (serum estradiol concentrations fell to menopausal levels).