A phase III multicentre study was carried out with the GnRH agonist buserelin (Suprecur) at five university institutions (Dept. of Obstetrics and Gynaecology of Essen, Kiel, Tübingen, Münster and Erlangen) to evaluate the effect of Buserelin (3 x 300 micrograms daily intranasal) on endometriosis for a period of 6 months. Endometriosis was verified by light microscopy in all cases. The data of 430 patients could be evaluated. The extent of the endometriotic lesions was studied before the start and at the end of therapy. The mean age of the patients was 30 +/- 5 years. For 300 patients (66.4%) sterility was a main problem. Previous treatment for endometriosis consisted of operation in 20.5%, treatment with danazol in 16.8% and treatment with gestagens in 11.6% of the patients. Under treatment FSH changed from 11.0 +/- 31.4 mIU/ml to 7.0 +/- 5.6 mIU/ml at the end of therapy; LH from 11.1 +/- 12.9 mIU/ml to 7.4 +/- 7.3 mIU/ml; estradiol from 113.9 +/- 89.0 to 35.9 +/- 60.3 pg/ml and progesterone from 7.2 +/- 11.2 ng/ml to 0.8 +/- 1.4 ng/ml. The effect on endometriosis was judged according to the revised American Fertility Society score (rAFS 1985). The changes of the rAFS-score stages were as follows (before/after therapy): Stage 0: 0%/42.8%; stage I: 43.5%/38.5%; stage II: 31.1%/11.2%; stage III: 18.1%/2.2%; stage IV: 6.3%/0.2%. The total mean AFS-stage changed from 1.9 to 0.7. The frequency of clinical symptoms improved (before/after therapy): dysmenorrhoea 65.0%/11.2%; pelvic pain 48.2%/14.1%; pain at the time of gynaelogical examination 40.0%/9.7%.(ABSTRACT TRUNCATED AT 250 WORDS)