We have previously reported reversible hypogonadism induced by the intranasal administration of the luteinizing hormone-releasing hormone agonist buserelin as a new therapeutic approach for endometriosis. Thirteen patients were randomized to receive intranasal buserelin (400 micrograms 3 times a day) or subcutaneous buserelin injection (200 micrograms once daily) for a 6- to 9-month period. Both routes of administration were effective in inhibiting serum estradiol levels to near the menopausal range after 1 month of treatment. The two dosage regimens had also a comparable efficacy in alleviating endometriosis symptoms and in reducing the revised American Fertility Society scoring at laparoscopic examination. The implant score mainly decreased by more than 70%. The occurrence of side effects was similar in both groups, and side effects were mainly hot flushes, dyspareunia secondary to decreased vaginal secretion, and decreased libido. Results of hemogram, urinalysis, and serum biochemical and hormonal tests remained in the normal range. The ovulatory cycle rapidly returned after the cessation of treatment, and three pregnancies occurred in six previously infertile patients. Intranasal and subcutaneous buserelin were well accepted and equally effective in inhibiting the pituitary-ovarian function, which caused mild menopausal symptoms but an important regression of endometriosis.