With the development of fetal ultrasonography, detection of fetal ovarian cysts has been increased. Although ovarian cyst formation during the perinatal period is a self limiting process, there is still considerable controversy regarding the best treatment of the fetal ovarian cyst. The purpose of this study is to evaluate the natural history of fetal ovarian cysts and to analyze the result of treatment. From 1995 to 2004, 31 consecutive fetuses with ovarian cysts were followed by ultrasonography during the perinatal period. The fetal ovarian cyst was diagnosed by prenatal ultrasonography between 25weeks and 38 weeks and the mean size of the cysts was 5cm (ranged from 2 to 8cm). At birth, 3 cysts disappeared. In 2 cases, the diagnoses were changed to multicystic kidney disease and intestinal duplication. During following up of 26 cysts, 15 cysts have resolved completely. Seven cysts required oophorectomy because of cyst torsion (n=3), differentiation of tumorous condition (n=2), increased size of cyst (n=1), and large size (8cm) of cyst at birth (n=l). Fetal ovarian cyst should primarily be observed, and only in the limited cases, surgical treatment would be required for the risk of complications such as torsion and differentiation from benign to malignant pathology.