A prospective multicenter trial has been commenced in Korea to investigate the treatment efficacy of the levonorgestrel-releasing intrauterine system in patients with endometrial hyperplasia. The levonorgestrel-releasing intrauterine system is an alternative to oral progesterone without the disadvantages of oral progestogens. Therefore, we hypothesize that if the therapeutic efficacy of the levonorgestrel-releasing intrauterine system is similar to or greater than that of oral progesterone, the levonorgestrel-releasing intrauterine system could become the standard treatment for endometrial hyperplasia patients who do not want a hysterectomy. The levonorgestrel-releasing intrauterine system is inserted into uteri of patients with histologically confirmed endometrial hyperplasia. An office endometrial aspiration biopsy and transvaginal ultrasound are conducted every 3 months at an outpatients clinic. The primary endpoint is the response rate. The secondary endpoint is to estimate the consistency of the results of the office endometrial aspiration biopsy during the levonorgestrel-releasing intrauterine system being placed in uterus and a dilatation and curettage procedure.