Aim: The levonorgestrel-releasing intrauterine (LNG-IUS) system is an effective primary treatment for adenomyosis; however, it has high expulsion rates. We aimed to modify the system-allowing affixion to the myometrium-and evaluate the expulsion rate, effectiveness, and side effects in patients with adenomyosis and heavy menstrual bleeding. Methods: This study included patients with adenomyosis and heavy menstrual bleeding who underwent implantation of: a modified LNG-IUS (experimental group, n = 47); and the original system after gonadotropin-releasing hormone agonist treatment (control group, n = 47), between January 2014 and April 2016. Results: In the experimental group, two device expulsions occurred 12-18 months postimplantation. In the remaining 45 patients, the system was safely removed after the 60-month validity period, and no extrauterine device movement or infection occurred. In the control group, downward displacement and expulsion of the device occurred in eight (17%) patients within 60 months. The 5-year total expulsion rates were 4.3% and 17.0% in the experimental and control groups, respectively (p = 0.045). There were significant changes in the pretreatment severity of dysmenorrhea, menstrual volume, uterine volume (cm 3 ), and hemoglobin level in each group compared with after 1 year (p < 0.01 in all groups). The severity of dysmenorrhea, menstrual volume, uterine volume, and hemoglobin level after 1 year were similar between the two groups (p > 0.05 in all groups). Conclusions: Use of the modified LNG-IUS is a safe, cost-effective, and simple method for reducing the downward movement and expulsion rate in patients with adenomyosis and heavy menstrual bleeding.