Luteinising hormone-releasing hormone agonist (Buserelin) therapy administered for a period of 6 months in 4 patients with longstanding, severe, danazol-resistant endometriosis, was found to be effective in reducing all complaints related to endometriosis. From 2 weeks on, nearly half of the E2 determinations were below the sensitivity level of the assay, while the other values were predominantly in the range of the early follicular phase. Side effects associated with the induced hypoestrogenemia were mild and well tolerated. After six months of follow-up without treatment, one patient who desired pregnancy conceived shortly after cessation of therapy and one patient showed lasting amelioration of her complaints. The symptoms relapsed in the other two, possibly due to inadequate dose and/or duration of treatment.