Inclusion Criteria

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