Discordancy in size between the umbilical arteries was detected during real-time ultrasonographic study of six patients. Cross-sectional and longitudinal views from multiple locations of the three-vessel umbilical cords documented the size discrepancies in a total of 23 serial examinations. The size difference was defined by cursor measurement of the artery diameters. Pulsed- and continuous-wave Doppler evaluation of the dissimilar arteries in each cord demonstrated discordant flow velocity waveforms. The mean difference between small and large artery systolic-diastolic ratios (S/Ds) was significant (P less than .0001). Two of the six patients studied had poor perinatal outcomes. Pathologic confirmation of the artery discordance was provided by gross and microscopic examination of an undrained cord segment from a seventh patient who had dissimilar artery sizes and S/Ds on ultrasonographic inspection. The finding of umbilical artery discordance has not been previously described in the obstetric ultrasound literature, and further investigation is warranted.