Few data on the long-term efficacy of intrauterine devices (IUD) are available, and this article reports on the final 12-year experience with the TCu220C and TCu380A devices from two randomized, multicenter trials conducted in 24 centers. A total of 3,277 and 1,396 women, respectively, were recruited for use of each device between 1981 and 1986 and followed at 3, 6, and 12 months after insertion and yearly thereafter. At the end of 12 years, a total of 17,098 women-years of experience had been accumulated for the TCu220C and 7,159 women-years for the TCu380A. The cumulative 12-year intrauterine pregnancy rates were 7.0 (standard error [SE] 0.6) per 100 women for the TCu220C and 1.9 (SE 0.5) for the TCu380A (p < 0.001). Pregnancy rates were highest in the first years after insertion; the TCu220C had a consistently higher annual pregnancy rate than did the TCu380A at all intervals since insertion. No pregnancies were reported with the TCu380A after 8 years of use. Total medical removals were approximately 6% in the first year and dropped to approximately 4% per year for each device for up to 12 years of use (cumulative 12 year rates were 37.3 [SE 1.3] and 40.2 [SE 2.1] per 100 women for the TCu220C and TCu380A devices, respectively). The overall continuation rate at all intervals since insertion was higher with the TCu220C device, mainly due to higher removal rates for nonmedical reasons with the TCu380A. The cumulative ectopic pregnancy rates were 0.7 and 0.4 for the TCu220C and TCu380A, respectively. Pregnancy rates were higher in the Chinese compared with the non-Chinese centers for both devices, though the greater efficacy of the TCu380A was apparent in both groups of centers. The total medical and nonmedical removal rates were lower in the Chinese compared with the non-Chinese centers, and did not show any substantial differences between the devices. We conclude that both devices are safe and effective for at least 12 years of use and the low pregnancy rate with the TCu380A is comparable with that reported in the United States among women who had undergone tubal sterilization. The very high efficacy of the TCu380A makes it the IUD of choice, and it can be considered as a potentially reversible, non-surgical alternative to sterilization for women requiring very long-term pregnancy protection.