A study of the United States acceptors of the Copper T, model TCu 380A, initiated in 1972, was carried to the completion of 4 years of observation. The 1051 receptors, whose median age was 22.3 years, accumulated a total of 1927 years of use. At the end of 4 years, the cumulative net pregnancy rate was 1.9 per 100 women, and the gross rate was 2.8 per 100. The annual continuation rate over the 4-year interval averaged 77 per 100. The TCu 380A intrauterine device (IUD) has provided extremely effective and undiminishing protection against pregnancy through 4 years of use in a group of young women. This degree of protection has not as yet been equaled when randomized comparative trials have been carried out with other copper-bearing IUDs.