Flow velocity waveforms were recorded by Doppler ultrasonography from the uterine arteries, umbilical artery, thoracic descending aorta, renal artery and middle cerebral artery in fetuses of 57 pregnancies complicated by preterm labor with intact membranes. The uterine artery resistance index was significantly increased in patients with preterm labor when compared to reference limits for gestation, but no relationship was found with the time interval between Doppler recordings and delivery. The group of fetuses (n = 15) delivered within 48 h from entry to the study showed significantly reduced pulsatility index values from the middle cerebral artery when compared to fetuses delivered later or to the normal reference limits for gestation. No significant differences were found in the other vessels studied. In conclusion preterm delivery is associated with alterations in fetal cerebral waveforms and knowledge of these changes may prove useful in the evaluation of patients with preterm labor.