Letrozole in the Treatment of Severe and Recurrent Endometriosis

Inclusion Criteria * Premenopausal women \> 18 years of age. * Laparoscopic and Histologically confirmed diagnosis of moderate or severe endometriosis according rASRM-Score III and IV * BMI less than 40 kg/m². * Patient is sexually abstinent or using mechanical (condoms, diaphragms) or sterilization methods of contraception and is willing to continue using them throughout the study. * Patient is willing and able to comply with study requirements. * Written informed consent. Exclusion Criteria * Endometriosis stage I-II acc. according to rASRM * Women with other causes of chronic pelvic pain including infectious, gastrointestinal, musculoskeletal, neurologic or psychiatric. * Significant abnormalities in the physical or laboratory examination including renal and liver function more than twice the normal range. * Patient desires pregnancy for the duration of the study, is pregnant or breast feeding. * GnRH therapy during the last 6 months * Use of hormonal contraception, selective estrogen receptor modulators, progestins, estrogens, steroids, or ovulation induction in the 4 weeks prior to inclusion into the study. * Untreated abnormal pap smear or other gynecologic condition. * History of venous thrombosis events including deep vein thrombosis, pulmonary embolism, or retinal vein thrombosis. * History of stroke, complicated migraine, or documented transient ischemic attack. * Known hypersensitivity to any ingredient of the study medication. * Treatment with other aromatase inhibitors * Other investigational drugs within the past 30 days and the concomitant use of investigational drugs. * History of non-compliance to medical regimens, and patients who are considered potentially unreliable. Additional protocol-defined inclusion / exclusion criteria may apply