The levonorgestrel intrauterine system (LNG IUS) releases 20 microg/24 h of levonorgestrel from a polymer cylinder mounted on a T-shaped frame and covered with a release rate-controlling membrane. It is approved for 5-year use. The most outstanding features of LNG IUS are its high contraceptive efficacy and reduction of menstrual blood flow. No single mode of action can account for its contraceptive efficacy. The endometrium becomes thin and inactive, and the cervical mucus turns scanty and viscous. Although ovulation may be disturbed to some degree, estradiol production continues normally. The Pearl index for LNG IUS from large clinical trials is 0.1. Extrauterine pregnancies occur in 1 in 5000 users per year. Both the volume of menstrual blood loss and the number of bleeding days are reduced. During the first year of use, 20% of women become amenorrheic. There is an initial increase in the mean number of bleeding and spotting days, but in 3 to 6 months the number of bleeding and spotting days is the same as observed in copper IUD-users. The variation between individuals is wide and unpredictable. There are also additional health benefits secondary to the inactivation of the endometrium: increased hemoglobin, decreased dysmenorrhea, a possible decrease in pelvic inflammatory disease. LNG IUS may also decrease the growth of fibroids. LNG IUS is well accepted by users, with typical annual continuation rates above 80% in clinical studies.