Fetal hematocrit values of blood obtained by percutaneous umbilical blood sampling were correlated with ultrasound findings in 35 samples from 15 pregnancies undergoing evaluation for Rh or Kell sensitization. Intravascular fetal transfusion was performed after a low hematocrit was obtained on 29 of 35 occasions. All fetuses with sonographic evidence of hydrops had a hematocrit of 15% or less, although three fetuses with hematocrits below 15% showed no signs of hydrops. In patients followed with serial sonography, hydramnios was noted as the earliest sonographic abnormality in six of nine pregnancies. All six fetuses were anemic (hematocrit 14-26%) and required transfusion. However, in three pregnancies where the fetus was anemic (hematocrit 22%), there was no hydramnios or other sonographic abnormality. Increased placental thickness was observed in association with fetal hydrops and a hematocrit below 15% in four cases, as well as in three other cases with fetal hematocrit between 16-29% but no fetal hydrops. Measurements of umbilical vein diameter provided no useful information because no increase was observed in these measurements, even in pregnancies with advanced fetal disease evidenced by hydrops.