Thirty simple adnexal cysts in postmenopausal women detected by sonography were followed prospectively. The cysts were totally anechoic or with at most two septations. Size varied from 2 cm to 8 cm, but one was as large as 17 cm. Only seven were discovered at the pelvic examination prior to the scan. Fifteen underwent surgery and no malignant tumor was found. The remaining 15 were followed by sonography only. Six lesions had disappeared after one month, altogether 12 had disappeared after seven months. After two years, only two 2-cm cysts remained. The results indicate that surgery on simple adnexal cysts may not be necessary but they should be followed by sonography.