A randomized comparative study of the Copper T 300, the standard Dalkon Shield® and the Silicone Shell Loop D was conducted on 776 parous women. Net cumulative first event rates at 1 year and at 18 months, calculated by life-table analysis, were as follows: Copper T 300—pregnancy 1.2/1.9, expulsion 6.4/7.0, removal for pain or bleeding 7.4/10.8; Dalkon Shield—pregnancy 2.7/4.5, expulsion 5.8/5.8, removal for pain or bleeding 7.7/14.7; Shell Loop—pregnancy 3.8/4.9, expulsion 0.5/0.5, removal for pain or bleeding 9.1/13.1. None of the differences in event rates, except the lower expulsion rate of the Shell Loop, were significant. The continuation rates for the three devices at both 12 and 18 months were similar, indicating that there was little difference in their performance. As the results of this study showed a similarity of the incidence of events among very differently designed IUD's, previously reported differences in performance of these and possibly other types of IUD's may very likely be more related to differences among clinics than to differences among the devices themselves.