The localization of uterine devices by ultra-sound techniques was investigated. Seven different types of intra-uterine contraceptive devices were checked under immersion in water and in the uterus of 585 wearers of I.U.D.'s. The quality of the ultra-sound pictures is mostly determined by the type of intra-uterine device. Under optimal conditions the investigated types of I.U.D.'s produced standard ultra-sound pictures which permitted their identification. The evaluation of the localization of the intra-uterine contraceptive devices in the uterus was reasonably accurate. At first the classification of normal localization, lower segment localization and cervical localization was used, and this was later enlarged by the measurement of the distance between the uterine fundus and the end of the device closest to the fundus. It was possible to estimate the contraceptive efficiency with these measurements. In 486 cases (83%) the intra-uterine device was found in normal position. In 24 (4.1%) of the cases removal and repeat insertion of the device was suggested. In 18 (3.1%) cases the device was not localized despite normal intra-uterine localization.