A prospective study was carried out from June 1973 to August 1976 in 131 women fitted with a newly developed IUD (Progestasert). The IUD's were removed as planned after 12 months with the exception of 4 cases in which they were retained for up to 17 months. The evaluation of the trial was performed according to the Life Table Analysis and the usual statistical methods. 1496 application months were evaluated. One case each of intrauterine and extrauterine pregnancies were observed. One patient experienced spontaneous expulsion of the IUD. In 9 cases medical reasons led to the removal of Progestasert at an earlier date. Duration, intensity, and interval of menstruation as well as frequency of spotting and dysmenorrhea were recorded. A slight increase in duration and interval of menstruation was seen whereas bleeding intensity was reduced. Increased spotting, especially during the first month after insertion, and a significant reduction of dysmenorrheal complaints were noted after 2 months. In addition to 8 removals due to bleeding disturbances, 6 other patients experienced spotting throughout the entire treatment period. Laboratory tests such as ESR, WBC, hemoglobin, hematocrit and cytological smear determinations were carried out. No pathological alterations of these parameters were found.