Although the pathophysiology of adenomyosis has not been clarified, it is thought to be related to ectopic endometrium, which depends on hormonal regulation. The levonorgestrel-releasing intrauterine system (LNG-IUS) is effective for the medical treatment of adenomyosis. However, the underlying molecular mechanisms by which LNG-IUS ameliorates adenomyosis pathology remain unclear. This study was designed to compare the expression levels of steroid receptor coregulators in human endometrium of control and participants with adenomyosis and to determine whether LNG-IUS modulated their expression. Immunohistochemistry with H-scores was performed. Steroid receptor coactivators were shown to have significantly decreased expressions at the secretory phase in the LNG-IUS group when compared to the other groups. Expression of transcriptional intermediary factor 2 was lower in the LNG-IUS group than in both the control group (P = .015) and the untreated adenomyosis group (P = .019) during the secretory phase. Amplified in breast cancer 1 expression was higher in the stromal cells of the untreated adenomyosis group than in those of the controls (P = .017) during the secretory phase; however, levels were lower in the LNG-IUS group (P = .005). Nuclear receptor corepressor expression increased during the proliferative phase and decreased during the secretory phase in untreated adenomyosis; this pattern was reversed in the control and LNG-IUS groups. Thus, an altered expression of steroid receptor coregulators may play a role in adenomyosis development and treatment.