This study was undertaken to evaluate the incidence and the factors associated with uterine bleeding while taking GnRH agonist treatment combined with tibolone add-back therapy in endometriosis patients of reproductive age.The medical records of 188 endometriosis (stages III–IV) patients of reproductive age receiving postoperative GnRH agonist treatment combined with tibolone add-back therapy for 6 months were reviewed. Clinical features were analyzed and compared between the two groups: Group A, patients without an episode of uterine bleeding (n=137) and Group B, patients that had experienced uterine bleeding (n=51).Demographic profiles were not different in the two groups. The incidence of uterine bleeding was 27.1% and irregular spotting was the most frequent bleeding pattern. The proportion of patients who received ovarian surgery during pelviscopy was higher in Group B (p<0.05), but other clinical characteristics were not different between two groups.The incidence of uterine bleeding was 27.1%. And, the proportion of patients who received ovarian surgery was higher in patients with uterine bleeding.