In a randomized study the clinical efficacy of the traditional intrauterine device. Copper-T-200 Ag was compared with the recently developed model Nova T. Altogether 93 women with regular menstrual cycles were examined; 45 women had a Nova T inserted and 48 the traditional Copper-T-200 Ag. The biosocial data of the women in the two groups did not differ from one another. No pregnancies occurred in either group within the first 12 months. Expulsion occurred in 2 women with Nova T and removal of the device had to be undertaken for bleeding and pain in 3 women with a Copper-T-200 Ag. At 12 months the two models did not differ in the number of recorded incidents. The reliability of randomized studies is discussed, as well as problems arising from the fact that the mode of insertion is not identical with both devices and requires practice in the case of the new model.