In a randomized comparative use-effectiveness study of the copper-releasing intrauterine devices (IUDs), Nova T and Copper T 200, an analysis was made of the differences in women between the participating countries and of the influence of various factors on pregnancy rate and rates of terminations. In a cohort of 1865 women the five-year gross cumulative pregnancy rates were 3.4 per 100 women for Nova T and 8.7 per 100 women for Copper T 200. The rate of unwanted pregnancy was significantly related to the type of IUD, the country, and the age of the women. Prognostic factors for expulsion or removal of the device were age, country, parity, previous use of IUD, previous vaginal delivery, and previous abortion. The differences among the results for the participating countries may reflect the effects of different policies in clinics.