Real-time ultrasound was a valuable adjunct in the management of 57 consecutive isoimmunized pregnancies. The pathophysiology of hydrops fetalis as it affects the sonographic anatomy of the fetus and placenta is discussed. Real-time ultrasound was slightly more accurate than amniotic fluid bilirubin analysis in predicting moderate to severe erythroblastosis fetalis. The most useful finding was a disproportionate growth of the fetal abdominal circumference in relationship to the biparietal diameter. Ultrasound proved invaluable in several difficult situations, such as contaminated amniotic fluid, ruptured membranes, and twins. Suggestions for the further quantitation of this diagnostic test are explored.