In 68 red blood cell isoimmunized pregnancies the mean velocity of blood in the fetal aorta was measured by pulsed Doppler ultrasonography, and the fetal hemoglobin concentration was determined in blood samples obtained by cordocentesis. The values were compared to reference ranges of mean aortic velocity and hemoglobin for gestation, which were determined from 218 218 pregnancies that were not complicated by fetal hemolysis. In the red blood cell isoimmunized pregnancies there was a significant positive correlation between the aortic mean velocity and the hemoglobin deficit for the nonhydropic fetuses and a significant negative correlation between these two parameters for the hydropic fetuses.