BACKGROUND This study was conducted to evaluate the effect of the levonorgestrel-releasing intrauterine system (LNG-IUS) on adenomyotic lesions diagnosed and monitored by magnetic resonance imaging (MRI). STUDY DESIGN LNG-IUS was inserted during menstrual bleeding in 29 women, 24 to 46 years of age, with MRI-diagnosed adenomyosis associated with menorrhagia and dysmenorrhea. Clinical evaluations were carried out at baseline and at 3 and 6 months postinsertion. MRI was performed at baseline and at 6 months postinsertion and was used to calculate junctional zone thickness (in mm), to define the junctional zone borders, to identify the presence of high-signal foci on T(2)-weighted images and to calculate uterine volume (in mL). RESULTS A significant reduction of 24.2% in junctional zone thickness was observed (p<.0001); however, no significant decrease in uterine volume was observed (142.6 mL vs. 136.4 mL; p=.2077) between baseline and the 6-month evaluation. A significant decrease in pain score was observed at 3 and 6 months after insertion (p<.0001); however, six women continued to report pain scores >3 at 6 months of observation. At 3 months of use, the most common bleeding pattern was spotting, and at 6 months of observation, oligomenorrhea was the most common pattern observed, although spotting was present in one third of the women. CONCLUSIONS The insertion of an LNG-IUS led to a reduction in pain and abnormal bleeding associated with adenomyosis. MRI was useful for monitoring response of adenomyotic lesions to the LNG-IUS.