BACKGROUND:Endometriosis is a common gynaecological condition that frequently presents with the symptom of pain. The precise pathogenesis (mode of development) of endometriosis is unclear but it is evident that endometriosis arises by the dissemination of endometrium to ectopic sites and the subsequent establishment of deposits of ectopic endometrium. The observation that endometriosis is rarely seen in the hypo-oestrogenic (low levels of oestrogen) post-menopausal woman led to the concept of medical treatment by induction of a pseudo-menopause using Gonadotrophin Releasing Hormone Analogues (GnRHas). When administered in a non-pulsatile manner (the pituitary is normally stimulated by pulses of natural GnRH and all analogues act on the pituitary at a constant level) their use results in down regulation (switching off) of the pituitary and a hypogonadotrophic hypogonadal state (low levels of female hormones due to non stimulation of the ovary). OBJECTIVES:To determine the effectiveness of Gonadotrophin Releasing Hormone analogues (GnRHas) in the treatment of the painful symptoms of endometriosis by comparing them with no treatment, placebo, other recognised medical treatments, and surgical interventions. SEARCH STRATEGY:The search strategy of the Menstrual Disorders and Subfertility review group (please see Review Group details) was used to identify all randomised trials of the use of GnRHas for the treatment of the painful symptoms of endometriosis. SELECTION CRITERIA:Trials were included if they were randomised, and considered the effectiveness of GnRHas in the treatment of the painful symptoms of endometriosis. DATA COLLECTION AND ANALYSIS:Twenty-six studies had data appropriate for inclusion in the review. The largest group (15 studies) compared GnRHas with danazol. There are five studies comparing GnRHas with GnRHas plus add-back therapy, three comparing GnRHa with GnRHa in a different form or dose, one compares them with gestrinone, one with the combined oral contraceptive pill, and one with placebo. Data was extracted independently by two reviewers. The authors of eleven studies have been contacted to clarify missing or unclear data. Only four have replied to date. Data on relief of pain, change in revised American Fertility Society (rAFS) scores, and side effects was collected. MAIN RESULTS:No difference was found between GnRHas and any of the other active comparators with respect to pain relief or reduction in endometriotic deposits. The side effect profiles of the different treatments were different, with danazol and gestrinone having more androgenic side effects, while GnRHas tend to produce more hypo-oestrogenic symptoms. AUTHORS' CONCLUSIONS:There is little or no difference in the effectiveness of GnRHas in comparison with other medical treatments for endometriosis. GnRHas do appear to be an effective treatment. Differences that do exist relate to side effect profiles. Side effects of GnRHas can be ameliorated by the addition of addback therapy.