Many investigators have attemped to explain the mechanism of contraceptive action of intrauterine device and the causation of side effects since the new plastic intrauterine devices have been widely used. Bleeding and pain are the most embarrassing side effects associ- ated with the use of IUD`s and they are the major reasons for discontinuation of use. How- ever, little is known about the causation of these side effects. In the previous study, which was undertaken by one of the authors to observe the beha- vior of a device in the uterine cavity and its possible relationship to the side effects, it was observed that the elongation of the Lippes` loop was the most frequently observed distorsion and this was associated with the occurrence of side effects, particularly with the intermenstrual bleeding. The present study was to investigate the possible relationship between the frequency of side effects and the distorsion or displacement of the device in connection with the shape and size of the uterine cavity. METHOD Two hundred and fourteen women who had Lippes` loop, mostly 27.5mm, were randomly selected from the Family Planning Clinic of Seoul National University Hospital and subjected to the hysterographic examination. All examinations include a 2-film bysterogram. The first film is taken after introducing 2 ml of water boluble HYPAQUE diluted to 2:3 with normal saline. This film shows the device clearly with some of the surrounding cavity of the uterus. Then 5ml of the same solution are added and the second film is taken for outlining the entire cavity. The uterus was always straightened by pulling on the cervix before the X-ray picture was taken. Supported in part by a grant of the International Planned Parenthood Federation