Since 1965 there has been a substantial increase in the number of women in the United States who use the intrauterine contraceptive device (IUD). A collaborative multicenter case--control study was conducted to examine uterine perforation and uterine incarceration as well as several other suspected complications related to use of an IUD. To determine which attributes of the IUD and which user characteristics contribute to the risk of uterine perforation and incarceration, the authors analyzed 32 women with uterine perforation requiring transperitoneal removal, 106 women with uterine incarceration of an IUD which was removed transcervically, and 497 controls. Most important, women who were lactating at the time of IUD insertion were 10 times as likely to have had a uterine perforation as women with at least 1 live birth but who were not lactating at the time of insertion. An incarcerated IUD resulting in a difficult removal was 2.3 times as likely among women lactating at the time of insertion compared to women not lactating at the time of insertion. The likelihood of both uterine perforation and uterine incarceration were unchanged regardless of the type of IUD used.