Objective: To investigate the changes of mestruation patterns and adverse effects during the treatment of levonorgestrel-releasing intrauterine system(LNG-IUS)for symptomatic adenomyosis in a prospective cohort study. Methods: From December, 2006 to December, 2014, patients of symptomatic adenomyosis diagnosed by transvaginal ultrasound in Peking Union Medical College Hospital were given LNG-IUS. Before and after placement of IUS, all patients' parameters were recorded, including carrying status of IUS, symptoms and scores of dysmenorrhea, menstruation scores, biochemical indicators, physical parameters, menstruation patterns and adverse effects. Risk factors for changes of menstruation patterns and adverse effects, and their impact on treatment effects were analyzed. Results: Totally 1 100 cases met inclusion criteria, with median age 36 years(range 20-44 years), median follow-up 35 months(range 1 -108 months). During follow-up changes of menstruation patterns increased significantly with amenorrhea and shortened-menstruation being the most common manifestations. On 3, 6, 12, 24, 36, 48 and 60 months after the placement of LNG-IUS, 0, 5.8%(43/744), 6.9%(47/682), 10.1%(60/595), 17.3%(87/502), 27.2%(104/383)and 29.6%(82/277)patients achieved amenorrhea respectively(P<0.01). Total and subclassification of adverse effects decreased significantly(P<0.01). Within 12 months and >12 months after placement, abdominal pain and body weight increasing ≥5 kg/year were the most common adverse effects. Changes of menstruation patterns, total and subclassifications of adverse effects were neither dependent on patient parameters, treatment modes and treatment effects, nor could predict future LNG-IUS carrying status(all P> 0.05). After taking out of LNG-IUS, most changes of menstruation and adverse effects disappeared. Conclusions: During the treatment of LNG-IUS for symptomatic adenomyosis, changes of menstruation patterns increase gradually with amenorrhea and shortened-menstruation being the most common manifestations, while adverse effects decrease significantly. Changes of menstruation patterns or adverse effects neither have any risk factor nor have impact on treatment effects.