The objective of this study was to evaluate the efficacy and performance, for up to 1 year, of an intrauterine system releasing 20 microg/day of levonorgestrel (LNG-IUS, Mirena) in the treatment of women with menorrhagia. It was a descriptive, prospective, non-comparative study. A 20 microg/day LNG-releasing-IUS was inserted on any day during bleeding to 44 women (between 24 and 49 years of age) who presented with menorrhagia after medical therapies had failed. Menstrual patterns were assessed, and hemoglobin concentrations were measured before LNG-IUS was inserted and at 3, 6, 9, and 12 months of use. The most common bleeding pattern at 3 months after insertion was spotting, and after 6, 9, and 12 months the majority of women presented with amenorrhea or oligomenorrhea. Three women requested removal of the LNG-IUS because of spotting, and six women expelled it spontaneously. Hemoglobin levels were improved from 102 g/L to 123 and 128 g/L at 3 and 12 months, respectively, after insertion of the LNG-IUS (p < 0.01). At 12 months 79.5% of participants continued the use of LNG-IUS. In conclusion, LNG-IUS was an effective treatment for three out of four women with menorrhagia and could be an alternative treatment for women with menorrhagia who are either contraindicated for or refuse hysterectomy or endometrial ablation.