Objective. To compare the efficacy of aromatase inhibitor vs. gonadotrophin-releasing hormone agonists in treating premenopausal women with uterine adenomyosis. Design. A prospective randomized controlled study. Setting. A university hospital and a private practice setting. Population. Thirty-two patients with uterine adenomyosis. Methods. Patients were randomly allocated to receive oral letrozole (2.5mg/day) or a subcutaneous gonadotropin-releasing hormone agonist (goserelin, 3.6mg) for 12 weeks. Uterine and adenomyoma volumes were determined at baseline and during treatment at four, eight and 12 weeks. Outcome measures. Measurements were performed at baseline and during treatment at four, eight 8 and 12 weeks, and mean values were calculated. Symptoms at the start and after 12 weeks were evaluated. Results. No significant differences in the total uterine size between the post treatment uterine volumes in the two groups (20.1, 15.4 and 13.0cm3 vs. 21.7, 15.1 and 11.7cm3, at four, eight and 12 weeks, respectively). Total adenomyoma volume decreased by 8.6, 29.7 and 40.9% vs. 5.7, 34.6 and 49.1% after four, eight and 12 weeks of treatment, in group A and B, respectively. Two patients became pregnant in group A during treatment. Conclusions. Aromatase inhibitors are as effective as gonadotropin-releasing hormone agonists in reducing adenomyoma volume and improving symptoms.