Objective To determine the effectiveness of the Copper T380A (CuT380A) intrauterine device (IUD) as an emergency contraceptive in women.Design Prospective, multicentre, cohort clinical trial.Setting Eighteen family planning clinics in China.Sample A cohort of 1963 women, aged 18-44 years, requesting emergency contraception within 120 hours of unprotected sexual intercourse.Methods Women requesting emergency contraception were followed at 1, 3 and 12 months after the insertion of CuT380A.Main outcome measures Efficacy of CuT380A as emergency contraception and for up to 12 months of postinsertion use. Insertion complication rates, reported side-effects and continuation rates at 12 months were also recorded.Results No pregnancies occurred prior to or at the first follow-up visit, making CuT380A 100% effective as emergency contraception in this study. The pregnancy rate over the 12-month period was 0.23 per 100 women. In all, 29 (1.5%) women experienced a difficult IUD insertion process, requiring local anaesthesia or prophylactic antibiotics. No uterine perforations occurred. The main side-effects were increased menstrual bleeding and menstrual disturbances. The 12-month postinsertion continuation rate was 94.0 per 100 woman-years.Conclusions CuT380A is a safe and effective method for emergency contraception. The advantages of CuT380A include its ability to provide effective, long-term contraception.