To evaluate the efficacy, safety and tolerance of the GnRH-analogue Buserelin in the treatment of endometriosis, we started a non-comparative study of 52 patients with various stages of endometriosis. The dosage of 900 mcg/d was administered 3 times daily intranasally for 6 months. The degree of the disease was evaluated before and at the end of treatment by pelviscopy and biopsy. A regression of the implant score was found in 88% of patients. The mean AFS score was reduced from 17.4 +/- 1.9 before medication to 7.2 +/- 8.2 after therapy. Endometriosis related complaints were significantly improved during treatment. After 6 months 75% of the patients were without lower abdominal pain, 98% without dysmenorrhoea, and 85% without dyspareunia. As a result of the induced low level of oestrogen, 60% of the patients claimed hot flushes, 12% sweating, reduced libido, and dry vagina. Bleeding (spotting, break-through bleeding, menstruation) occurred in 40% during the first month, and was continuously reduced during the following period. Before, during and after therapy, venous blood samples were drawn to check laboratory tests for blood count, clotting parameters, clinical chemistry, serum electrolytes, as well as liver and lipid metabolism. All values stayed within the normal range without significant changes. In the follow-up period of at least 12 months, we achieved an uncorrected pregnancy rate of 41% in the group of 34 women with primary or secondary infertility.