The nylon T IUD is identical to the copper T 200 except for the composition of nylon/copper. A study conducted at the Family Planning Institute in Ljubljana, Yugoslavia compared the nylon T with the copper T. Devices were randomly administered to women who had not been pregnant during the month prior to insertion. The pregnancy rate for the nylon T group was 13.0 at 12 months, significantly higher (Por= 0.01) than the rate of 0.0 for the copper T group. Effectiveness of copper devices is related to the copper itself, rather than being an effect of the increased surface area alone.