A decrease in amniotic fluid in prolonged pregnancy is associated with increased fetal morbidity. However, few investigations have been reported on the prediction of this condition. Fetal renal arterial pulsatility index (PI), hourly fetal urine production rate (HFUPR) and amniotic fluid index (AFI) at 39 weeks were studied in 51 singleton pregnancies with (n = 14) and without oligohydramnios (AFI < 5, n = 37) in prolonged pregnancy. There was no difference in the average fetal renal arterial PI and HFUPR between the oligohydramnios group and the normal group. However, the average AFI in the oligohydramnios group was 7.5 +/- 0.9, which was lower than that in the normal group (p < 0.05). We speculated that AFI value at 39 weeks is useful for predicting the incidence of oligohydramnios in prolonged pregnancy.