Doppler blood cell velocities were measured in the aortas, inferior vena cavas, and umbilical veins of fetuses from isoimmunized pregnancies and related to the hematocrit levels of the fetal blood determined at fetoscopy. Pourcelot Index of flow in the umbilical artery was similarly studied. The mean velocities in the descending aortas and the Pourcelot Indexes of the umbilical arteries of both normal and affected fetuses correlated with fetal age. These velocities and indexes of affected fetuses also correlated inversely with the fetal hematocrit levels independently of the correlation with fetal age. The affected fetuses had higher mean velocities in the aorta and in the inferior vena cava than did normal fetuses. A simple model of multiple regression predicted the fetal hematocrit levels with a mean error of 3.8 hematocrit units (volume %).