International data on sterilized patients collected by the International Fertility Research Program (IFRP) were examined to determine the incidence of uterine perforation due to a sterilization procedure. Data on 16379 patients who underwent laparoscopic sterilization and 4378 who underwent minilaparotomy sterilization were reported to the IFRP through 18 medical centers from 11 countries during the January 1974 to October 1976 period. All the centers used standardized record forms and protocols. A total of 38 uterine perforations were identified: 29 among the laparoscopic sterilization group and 9 among the minilaparotomy group representing a rate of 1.8 and 2.1/1000 procedures respectively. Each patient who sustained a uterine perforation (case) was matched with 2 women who had no identified uterine perforation (controls) by center operator and route of approach. Control patients had undergone sterilization on a date close to that of their counterpart cases. 9 cases were omitted. All of the remaining 29 triplets were sterilized at least 6 weeks after termination of their last pregnancy. There were no statistically significant differences between cases and controls as to the following characteristics: race; age; parity; type of termination of last pregnancy; history of abdominal and pelvic operations; preexisting phase when sterilization was performed. 20 of the 29 cases and 21 of the 58 controls had an interval between termination of last pregnancy and date of sterilization of less than 1 year; the difference was statistically significant. Patients with an interval of less than 1 year were 4.8 times more likely to incur uterine perforation.