Measurement of the head-to-abdomen (H/A) ratio for differentiating between symmetrical and asymmetrical IUGR may be difficult in the presence of marked oligohydramnios. A total of 76 cases of IUGR with various degrees of oligohydramnios was studied. Sixteen (59%) out of 27 structural defects represented bilateral renal agenesis, 11 of which were diagnosed prenatally. When only the H/A ratio was measured (n = 29) the sensitivity in picking up a structural defect was 50%. Calculation of the pulsatility index in the umbilical artery and fetal internal carotid artery (n = 47), whether or not combined with amnioinfusion (n = 5), resulted in a sensitivity of 76%. It is suggested that the latter two techniques may provide valuable additional information as to the cause of IUGR.