Ten patients suffering from endometriosis were treated with danazol at a dose of 200 mg three times daily for 6 months. A thyrotropin-releasing hormone (TRH) test was carried out in the follicular phase before treatment, at the sixth month of treatment and after reappearance of the second menses. There was no statistically significant change in the basal level of serum thyroid stimulating hormone (TSH) during danazol treatment. On the contrary, the response of serum TSH in the TRH test was significantly higher during danazol treatment than before or after treatment. Our results support the view that danazol may reduce the secretion of hypothalamic TRH.