The present paper combines the estimates of efficacy and side effects of 10 mg mifepristone for emergency contraception obtained from randomized trials. A total of 6083 women participating in 12 randomized trials and receiving 10 mg mifepristone for emergency contraception up to 120 h after intercourse, were analyzed for efficacy. Between 4188 and 5833 women were analyzed for side effects and 3601 for delay of menses of more than 7 days. Prevented fractions, the effect of delay and of further acts of intercourse after treatment administration were analyzed in 3440 women, using individual data. The combined pregnancy rate from all the 12 trials was 1.7% [101/6083, 95% confidence interval (CI): 1.3-2.2]. From the three trials providing individual data, the combined pregnancy rate was 1.3% (45/3440, 95% CI: 0.9-1.7) and the estimate of pregnancies prevented was 83.4% (95% CI: 77.4-87.8). There was a sharp decline in efficacy when treatment was administered during the 5th day after intercourse compared to administration during the 1st day, the odds of pregnancy increasing by a factor of 5.3 (95% CI: 1.9-14.9). The relative risk of pregnancy was about 28 times higher among women with unprotected acts of coitus between treatment administration and the onset of next menses, compared with women reporting none [odds ratio (OR) = 27.6, 95% CI: 12.7-60.2]. The increase in risk for women reporting protected acts of intercourse during this interval was not statistically significant (OR = 1.8, 95% CI: 0.9-3.8). There was a large heterogeneity among trials in all side effects and delay of menses of more than 7 days (all had p < 0.0001 for the test of homogeneity). The percentage of women with nausea ranged from 0.0-19.4% (highest upper 95% confidence limit: 23.0%), that of vomiting from 0.0-4.3% (highest upper 95% confidence limit: 6.1%), that of lower abdominal pain from 4.3-19.1% (highest upper 95% confidence limit: 22.7%). The percentage of women with delay of menses of more than 7 days ranged from 4.3-25.8% (highest upper 95% confidence limit: 34.1%). We conclude that 10 mg mifepristone is an effective emergency contraception regimen, with an acceptable side-effects profile. Postponing treatment until the 5th day seriously decreases efficacy. The risk of pregnancy is dramatically increased among women having unprotected acts of intercourse between treatment administration and the onset of next menses. This risk may be enhanced for women whose ovulation is postponed by treatment.