This study was undertaken to evaluate the efficiency of ultrasonography for the prenatal assessment of Polyhydramnios. From May 1988 to October 1993, 2,320 subjects had single gestations undergoing fetal testing after 20 gestational weeks at the Severance Hospital. The incidence of Polyhydramnios diagnosed by amniotic fluid index was 6.8%(157/ 2,320). Maternal diabetes mellitus and fetal congenital anomalies diagnosed by ultrasonography were found in 6(3.8%) and 73(46.5%) patients among the polyhydramnios group. The causative factor of the other cases(49.7%) was unknown. The most common fetal malformations were central nervous system(26.0%) and gastrointestinal system anomalies(24.7 %). And they were followed by abdominal wall defect(8.2%), skeletal dysplasia(8.2%), cardiac lesion(6.8%), facial lesion(5.5%), and thoracic or diaphragmatic lesion(4.5%). The incidence of Cesarean section due to fetal distress, low 5-min Apgar score, large for getational age, admission to intensive care nursery, and perinatal mortality was not significantly different in the idiopathic polyhydramnios group compared with that of the control group(p <0.05). We found ultrasonic examination seems to be valuable diagnostic tool to assess the amount of amniotic fluid volume and detect fetal anomalies. Also it might be essential to perform the follow-up examination in idiopathic polyhydramnios to predict the perinatal outcome.