A prospective randomized study was conducted at the Family Planning Association of Hong Kong to compare the efficacy of the Yuzpe regimen and levonorgestrel (0.75 mg for two doses 12 h apart) in post-coital contraception. A total of 424 subjects were recruited into the Yuzpe group and 410 subjects into the levonorgestrel group; 77 subjects in the Yuzpe group and 79 subjects in the levonorgestrel group had further acts of intercourse during the treatment cycle. Fifteen pregnancies (3.5%) occurred in the Yuzpe group and 12 pregnancies (2.9%) in the levonorgestrel group. After excluding the patients who had further acts of intercourse, the failure rates in the Yuzpe group and the levonorgestrel group were 2.6 and 2.4% respectively. The incidence of nausea, vomiting and fatigue in the Yuzpe group was significantly higher than those in the levonorgestrel group. We conclude that levonorgestrel is an effective drug for post-coital contraception with a lower incidence of side-effects than the Yuzpe regimen.