Between December 1981 and June 1986, 52 women examined by ultrasonography were found to have clear pelvic cysts more than 5 cm in average diameter, without septa or solid areas. The cysts were persistent and did not respond to contraceptive pills for four to eight weeks. In all cases, there were histologic or cytologic results from surgical specimens or fluid aspirated from the cysts. In 15 women, the cysts were aspirated and revealed no malignant cells. Thirty-seven women underwent laparotomy, of whom 11 (29%) were found to have benign ovarian neoplasms (nine cystadenomas and two dermoid cysts). Because benign ovarian neoplasms are potentially malignant, and because a large number was found in our study, we recommend removal of the cysts by excision rather than aspiration.