The purpose of this study was to provide revised estimates of the effectiveness of the Yuzpe method of emergency contraception. Through a literature search, we identified eight studies that present the number of women treated and outcome of treatment by cycle day of unprotected intercourse relative to expected day of ovulation. Using five sets of external estimates of conception probabilities by cycle day of intercourse among women not using contraception, we assessed the effectiveness of the Yuzpe regimen. The 45 estimates of effectiveness, based on eight separate studies and the eight studies combined and five different sets of conception probabilities by cycle day, ranged from a low of 56.4% to a high of 89.3%. Our preferred point estimate is that the Yuzpe regimen reduces the risk of pregnancy by 74.1%, with a 95% confidence interval extending from 62.9% to 79.2%. True effectiveness is likely to be > 74% because treatment failures (observed pregnancies) include women who were already pregnant when treated and women who became pregnant after being treated.