To investigate the effects of the LH-RH agonist Buserelin [D-Ser (But)6 des-Gly10-LHRH ethylamide] on endometriosis, 64 patients were treated with 900 micrograms/d Buserelin intranasally over 6 months after histological verification of the disease. As shown by the follow-up operation at the end of treatment, 73% of cases showed regression of implants, whereas adhesions seemed to be unaffected. The uncorrected pregnancy rate of the 45 patients with a history of infertility was 40%, while the overall recurrence rate--confirmed by histological examination--was 9.4%. The endocrine parameters demonstrated a highly significant suppression of estradiol (E2) and a sharp decline of progesterone (Prog), indicating anovulatory cycles. Follicle-stimulating hormone (FSH) was unchanged, while luteinizing hormone (LH) and prolactin (Prl) decreased significantly. The androgenic parameters testosterone (T), dehydroepiandrosterone sulfate (DHEA-S), and sex-hormone-binding globulin (SHBG) revealed no relevant changes. Influence on bone metabolism could not be detected by measuring calcitonin and parathyroid hormone fragments (PTH-C and PTH-MM). Negative metabolic effects were absent in terms of hematology, clotting system, liver enzymes, renal parameters and lipid metabolism. Remarkable was a significant increase of high-density-lipoprotein cholesterol (HDL). Subjective complaints were mostly attributed to the therapy-induced hypoestrogenism. We consider Buserelin to be an effective drug in the treatment of endometriosis, with a low incidence of relevant side effects.