The purpose of this investigation was to compare the efficacy of conventional ultrasonographic measurements (i.e. head to abdomen circumference ratio, amount of amniotic fluid) with Doppler ultrasonography in the differential diagnosis of small-for-gestational age (SGA) fetuses. Blood flow velocity waveforms were recorded from the umbilical artery descending aorta and internal carotid artery in 121 intrauterine growth-retarded fetuses and the pulsatility index was evaluated as an index of vascular impedance. Conventional ultrasonographic measurements were of limited usefulness in the differential diagnosis, whereas the ratio between the pulsatility indexes from umbilical and carotid arteries proved to be a good index for discriminating SGA fetuses due to low growth potential (congenital infections, structural anomalies, chromosomal abnormalities, constitutional factors) from those caused by placental dysfunction (specificity 96.6%, sensitivity 89%, positive predictive value 98.7%, negative predictive value 74.3% and accuracy 90.9%.