The levonorgestrel-releasing intrauterine system (LNG IUS) is an effective method for contraception. It has a strong antiproliferative action on the endometrium. The endometrium is transformed under the influence of local levonorgestrel and becomes unresponsive to ovarian estrogens. This process is associated with progressive reduction of menstrual blood loss and menstrual duration. Scanty and irregular bleeding and/or spotting is usual during the first 3 to 4 months. The reduction of menstrual blood loss continues and after the first 9 months many women have no bleeding at all. However, they have normal ovarian function. The absence of bleeding is a result of the local antiproliferative action of the LNG IUS on the endometrium, which is also responsible for many health benefits during the use of this method. As with oral contraceptives, the risk of pelvic inflammatory disease is reduced, because of reduced menstrual blood loss, endometrial suppression, and thickening of the cervical mucus. There are some steroidal side effects: mood changes, oily skin, and acne. Weight increase is similar to that associated with copper intrauterine devices: 500 g per year over 5 years. Users should be told that the LNG IUS does not prevent sexually transmitted infection, and therefore women at risk should also use condoms for their protection.