Endometriosis is a chronic inflammatory disease that affects approximately 10-15% of women of reproductive age. Symptoms include dysmenorrhoea, chronic pelvic pain, dyspareunia and infertility. Removal of the endometriotic cyst (chocolate cyst) by surgery is a well-established treatment for symptomatic relief. However, recurrence of endometriotic cyst after surgical removal of the cyst is up to 30-50% after ovarian surgery. Oral contraceptive pills for 18-24 months after the surgery is widely used as a postoperative hormonal therapy because it has been shown to reduce the chance of recurrence of the endometriotic cyst, but recurrence is still high even after taking oral contraceptive pills.

Letrozole is an aromatase inhibitor. There are some preliminary reports that letrozole can cause shrinkage of endometriotic cysts and improve endometriosis-related pelvic pain by reducing oestrogen level, inflammation and stem cell recruitment that may be important in recurrence of endometriotic cyst. This is a randomized double-blinded placebo-controlled trial. The aim of this study is to assess whether taking letrozole in addition to oral contraceptive pills in the first 6 months after laparoscopic surgery (key-hole surgery) to remove the endometriotic cyst can reduce the risk of recurrence compared to oral contraceptive pills alone. The study also involves laboratory parts from a small portion of the endometriotic cyst specimens (removed during laparoscopy ovarian cystectomy) and endometrial biopsy (if the patient agrees) to assess the role of stem cells in the pathogenesis of endometriotic cysts.

Inclusion Criteria:

• Women aged 18-45 years old
• Scheduled to have laparoscopic ovarian cystectomy
• Unilateral or bilateral endometrioma on pre-operative ultrasound and confirmed by histology

Exclusion Criteria:

• Use of long-acting hormonal therapy in the 3 months before inclusion in the study
• Histological report of laparoscopic ovarian cystectomy showed atypical endometrioma
• Complex surgery including resection of deep infiltrating disease, bowel resection or hysterectomy
• Incomplete excision of endometrioma/ incision or drainage rather than ovarian cystectomy
• Suspicion of malignancy
• Contraindications to combined oral contraceptive pills, including: uncontrolled hypertension (systolic > 160mmHg or diastolic > 100mmHg), diabetes with retinopathy/ nephropathy/ neuropathy, current or past history of venous thromboembolism, ischemic heart disease, history of cerebrovascular accident, migraine with aura, severe liver disease, oestrogen-sensitive cancers, undiagnosed abnormal uterine bleeding, smokers (> 15 cigarettes/day) aged 35 years or above, or having body mass index >=35 kg/m2
• Planning to get pregnant in the coming 1 year
• Refusal to join the study