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