Ultrasonographic measurements of placental thickness, extrahepatic and intrahepatic umbilical vein diameters, abdominal circumference, head circumference, head/abdominal circumference ratio, and intraperitoneal volume were made in 50 rhesus-isoimmunized pregnancies at 18 to 26 weeks' gestation. The severity of fetal anemia was assessed by fetal blood sampling. Results in the isoimmunized group were compared with a control population of 410 normal pregnancies at 17 to 32 weeks' gestation. In the absence of fetal hydrops, none of the parameters studied could reliably distinguish mild from severe fetal hemolytic disease.