A screening visit,will be performed and written informed consent will be obtained from the patient. In addition the patient's medical history will be checked, vital signs recorded, pain and general health questionnaires completed, blood samples collected and any prior or concomitant medications will be noted. Bone density will be determined using a DEXA scan performed in the interval between the screening and baseline visits. One month after the screening visit, patients will return to the clinic for the baseline visit. At this visit a physical examination and urine pregnancy test will be performed, vital signs measured, specific validated questionnaires on pain and endometriosis related health will be completed and symptoms of oestrogen deficiency will be documented. Patients who are still considered to be eligible for the trial will receive an injection of Decapeptyl SR 11.25 mg, and will be dispensed sufficient Livial 2.5 mg tablets to last until the next study visit. Patients will return for repeat Decapeptyl SR injections every 3 months until Month 21 at which time the last Decapeptyl SR injection will be administered. At these visits patients will also be dispensed further supplies of Livial.