目的　比较米非司酮 2 5mg和 10mg用于紧急避孕的有效性、副反应及可接受性。 方法　采用双盲随机比较试验方法 ,共接纳合格妇女 2 0 0例 1次无防护性交后 72小时内随机进入Ⅰ组 (n =10 0 )和Ⅱ组 (n =10 0 ) ,予以治疗。Ⅰ组一次性服用米非司酮 1片 (每片 2 5mg)及安慰剂 1片 ;Ⅱ组一次性服用米非司酮 2片 (每片5mg) ,总量为 10mg。于预期下次月经日期的第 7天随访 ,了解避孕效果、副反应及月经的变化。 结果　两组各发生 1例妊娠 ,失败率均为 1% ,避孕有效率分别为 89 6 5 %和 90 40 % ,P >0 0 5 ,无统计学差异 ;各种副反应发生率较低 ,均在 10 %以内 ,两组相近 ;月经趋于正常者两组分别为 86 %和 88% ,P >0 0 5。结论　低剂量米非司酮 2 5mg ,乃至 10mg用于紧急避孕同样是安全有效的。 Objectives To compare the efficacy, side effects and acceptability of mifepristone 25 mg and 10 mg for emergency contraception. Methods A double-blind randomized trial was conducted. Patients in group I (n = 10 0) and group Ⅱ (n = 10 0) were randomized to receive treatment within 200 hours of unprotected intercourse with 200 women. In group Ⅰ, one tablet of mifepristone (25 mg each) and one tablet of placebo were taken at one time. In group Ⅱ, two tablets of mifepristone (5 mg each) were taken at a time for a total of 10 mg. Follow-up on Day 7 of the expected next menstrual period to understand contraceptive effects, side effects and menstrual changes. Results 1 case of pregnancy occurred in both groups, the failure rate was 1%, the contraceptive effective rates were 89 6 5% and 90 40%, respectively, P 0 05, with no significant difference; the incidence of various side effects was low, Were within 10%, the two groups were similar; normal menstruation were 86% and 88% of the two groups, P> 0.05. Conclusion Low doses of mifepristone 25mg, and even 10mg for emergency contraception is also safe and effective.