BACKGROUND: In the 1970s a new approach to the delivery of hormonal contraception was researched and developed. It was suggested that the addition of a progestogen to a non-medicated contraceptive device improved its contraceptive action. An advantage of these hormonally impregnated intrauterine systems (IUS) is that they are relatively maintenance free, with users having to consciously discontinue using them to become pregnant rather than taking a proactive daily decision to avoid conception. OBJECTIVES: To assess the contraceptive efficacy, tolerability and acceptability of hormonally impregnated intrauterine systems (IUSs) in comparison to other reversible contraceptive methods. SEARCH STRATEGY: Literature was identified through database searches, reference lists and individuals/organisations working in the field. Searches covered the period from 1972 to November 2003. SELECTION CRITERIA: All randomised controlled trials comparing IUSs with other forms of reversible contraceptives and reporting on pre-determined outcomes in women of reproductive years. The primary outcomes were pregnancy due to method/user failure and continuation rate. DATA COLLECTION AND ANALYSIS: The quality assessment of studies and data extraction were completed independently by two blinded reviewers. A quality checklist was designed to identify general methodological and contraceptive specific factors which could bias results. Events per women months and single decrement life table rates were extracted where possible for pregnancy, continuation, adverse events and reasons for discontinuation. Events per total number of women at follow up were collected for hormonal side effects and menstrual disturbance. When appropriate, data were pooled at the same points of follow up to calculate rate ratios in order to determine the relative effectiveness of one method compared to another. For the single decrement life table rates, the rate differences were pooled to determine the absolute difference in effectiveness of one method compared to another. Interventions were only combined if the contraceptive methods were similar. Non-hormonal IUDs were divided into three categories for the purpose of comparison with IUSs: IUDs >250mm2 (i.e. CuT 380A IUD and CuT 380 Ag IUD), IUDs <=250mm2 (i.e. Nova-T, Multiload, CuT 200 and CuT 220 IUDs) and non-medicated IUDs. MAIN RESULTS: Twenty-one RCTs comparing hormonally impregnated IUSs to a reversible contraceptive method met the inclusion criteria and it was possible to include eight of these in the meta-analyses, four comparing LNG-20 IUSs with non-hormonal IUDs, one comparing the LNG-20 IUS with Norplant-2 and three comparing Progestasert with non-hormonal IUDs. No significant difference was observed between the pregnancy rates for the LNG-20 users and those for the IUD >250mm2 users. However, women using the LNG-20 IUS were significantly less likely to become pregnant than those using the IUD <=250mm2. Women using the LNG-20 IUS were more likely to experience amenorrhoea and device expulsion than women using IUDs >250mm2. LNG-20 users were significantly more likely than all the IUD users to discontinue because of hormonal side effects and menstrual disturbance, which on further breakdown of the data was due to amenorrhoea. When the LNG-20 IUS was compared to Norplant-2, the LNG-20 users were significantly more likely to experience amenorrhoea and oligomenorrhoea, but significantly less likely to experience prolonged bleeding and spotting. No other significant differences were observed. Progestasert users were significantly less likely to become pregnant and less likely to continue on the method than non-medicated IUD users after one year, but no significant difference was noted for these two outcomes when Progestasert users were compared to IUD<=250mm2 users. The only other significant differences found in the meta-analyses were that Progestasert users were less likely to expel the device and more likely to discontinue the method because of menstrual bleeding and pain than users of IUDs <=250mm2. REVIEWERS' CONCLUSIONl the device and more likely to discontinue the method because of menstrual bleeding and pain than users of IUDs <=250mm2. REVIEWERS' CONCLUSIONS: Current evidence suggests LNG-20 IUS users are no more or less likely to have unwanted pregnancies than IUD >250mm2 and Norplant-2 users. The LNG-20 IUS was more effective in preventing either intrauterine or extrauterine pregnancies than IUDs <=250mm2. The contraceptive effectiveness of Progestasert was significantly better than non-medicated IUDs, but no difference was observed when compared to IUDs<=250mm2. Continuation of LNG-20 IUS use was similar to continuation of the non-hormonal IUDs and Norplant-2. Amenorrhoea was the main reason for the discontinuation for the LNG-20 IUS and women should be informed of this prior to starting this method.