Background: Effectiveness of Anastrozole and levonorgestrel-releasing intrauterine device (LNG-IUD, Mirena®) in the treatment of endometriosis. Methods: Randomized clinical trial. Elegibility criteria: Endometriomas >3×4 cm, CA-125>35 U/mL and symptoms suggestive of endometriosis. Thirty-one women were randomized to anastrozole+Mirena®+Conservative Surgery(CS) (n=8), anastrozole+Mirena®+transvaginal ultrasound-guided puncture-aspiration(TUGPA) (n=7), Mirena®+CS (n=9), or Mirena®+TUGPA (n=7). Interventions: Anastrozole 1 mg/day and/or only Mirena® for 6 months. CS or TUGPA one month after starting medical treatment. Results: A significant improvement in symptoms during the treatment (difference of 43%, 95% CI 29.9-56.2) occurred, which was maintained at 1 and 2 years. It was more significant in patients treated with anastrozole. For CA-125, the most significant decrease was observed without anastrozole. After CS for endometriosis, a reduction of findings of endometriomas and long-term recurrences occurred, with or without anastrozole, although anastrozole seems to delay recurrences. At 4,2±1,7 years, 88% of the patients who underwent CS were asymptomatic, compared to only 21% if TUGPA was performed, with or without anastrozole (p=0.019). Conclusion: Dosing anastrozole for 6 months, starting one month before CS of endometriosis, reduces more significantly the painful symptoms and delays recurrences, but has no other significant advantages over the single insertion of LNG-IUD (Mirena®) during the same time.