BACKGROUND: A previous randomised study suggested that the progestagen, levonorgestrel, given alone in two separate doses each of 0.75 mg caused nausea and vomiting in fewer women and might be more effective than the Yuzpe regimen of combined oral contraceptives for emergency contraception, although the difference was not significant. We compared these two regimens when started within 72 h of unprotected coitus. METHODS: We enrolled in the double-blind, randomised trial 1998 women at 21 centres worldwide. Women with regular menses, not using hormonal contraception, and requesting emergency contraception after one unprotected coitus, received levonorgestrel (0.75 mg, repeated 12 h later) or the Yuzpe regimen (ethinyloestradiol 100 microg plus levonorgestrel 0.5 mg, repeated 12 h later). FINDINGS: Outcome was unknown for 43 women (25 assigned levonorgestrel, 18 assigned Yuzpe regimen). Among the remaining 1955 women, the crude pregnancy rate was 1.1% (11/976) in the levonorgestrel group compared with 3.2% (31/979) in the Yuzpe regimen group. The crude relative risk of pregnancy for levonorgestrel compared with the Yuzpe regimen was 0.36 (95% CI 0.18-0.70). The proportion of pregnancies prevented (compared with the expected number without treatment) was 85% (74-93) with the levonorgestrel regimen and 57% (39-71) with the Yuzpe regimen. Nausea (23.1 vs 50.5%) and vomiting (5.6 vs 18.8%) were significantly less frequent with the levonorgestrel regimen than with the Yuzpe regimen (p<0.01). The efficacy of both treatments declined with increasing time since unprotected coitus (p=0.01). INTERPRETATION: The levonorgestrel regimen was better tolerated and more effective than the current standard in hormonal emergency contraception. With either regimen, the earlier the treatment is given, the more effective it seems to be.