Abstract Objectives: Lymphangiogenesis may be involved in the pathogenesis of adenomyosis. We investigated the lymphatic vessels of patients with adenomyosis, including those treated with levonorgestrel-releasing intrauterine system (LNG-IUS). Methods: Full-thickness uterine samples were obtained from patients who received hysterectomies. Twenty-one patients with adenomyosis and 17 patients with adenomyosis who were treated with LNG-IUS were included. Eighteen patients with cervical intraepithelial neoplasia served as controls. Immunohistochemical staining was performed with antibodies against podoplanin and lymphatic vessel endothelial hyaluronan receptor 1. The lymphovascular density (LVD) was analyzed in each sample by the “hot spot” method. Results: The LVDs were significantly higher in the endometrial and myometrial tissues of patients with adenomyosis compared with those of patients treated with the LNG-IUS or controls. No significant differences were noted between the LNG-IUS–treated group and controls. Evaluation of the LVDs according to the menstrual cycle showed that the differences in the endometrial tissues of the adenomyosis group and those of the LNG-IUS–treated group or the controls were more prominent during the secretory phase. Conclusions: Treatment with the LNG-IUS resulted in reduced lymphangiogenesis and LVD in the endometrial and myometrial tissues of patients with adenomyosis. Reduced lymphangiogenesis may be one mechanism by which the LNG-IUS reduces adenomyosis-related symptoms.