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. © RCOG 2010 BJOG An International Journal of Obstetrics and Gynaecology.