In 25 pairs of twins delivered by cesarean section, the flow velocity waveform in the umbilical artery was measured and the resistance index calculated. Umbilical artery and vein blood gases were measured at delivery. Results were tabulated by taking the difference in resistance indices between each fetus and plotting these values against the pO2-differences. The data revealed that index differences of more than 10 were of little value in detecting a pO2-difference of > 1 kPa, despite a sensitivity of 100%. The predictive value was only 41%, and specificity 44%. It was obvious, however, that a pO2-difference > 1 kPa occurred only when the resistance index was 76% or more in one fetus and below 76% in the other. The sensitivity of this index parameter was 100%, predictive value 64%, and specificity 78%. In conclusion, the pO2-level in a fetus seems to be unaffected by the impedance in umbilical circulation as long as the resistance index is below 76%. Above this value, there is a significant risk for fetal hypoxaemia.