Objective: To evaluate the fetal middle cerebral artery Doppler waveform for the prediciton of anemia in the RhD-alloimmunized fetus. Study Design: Doppler velocimetry of the fetal middle cerebral artery peak systolic velocity was measured in 52 non-hydropic, RhD-alloimmunized fetuses who underwent 103 cordocenteses. Normal values were obtained from 70 normal cases. The peak systolic velocity values were expressed as multiples of the median (MoM) for gestation. Hemoglobin threshold for developing hydrops was developed front 22 RhD-alloimmunized hydropic fetuses and severe anemia was defined as Hb less than or equal to 0.60 MoM. The most efficient threshold values fur the prediction of severe anemia in groups with no prior transfusion and those with prior transfusion were obtained by constructing ROC curves. Results: The mean gestational age (+/-S.D.) at cordocentesis was 28.0 +/- 4.6 weeks. Severe anemia was noted in 53 (51.5%) occasions. At a threshold middle cerebral artery peak systolic velocity value of greater than or equal to1.35 MoM, the sensitivity for severe anemia detection was 100% with a false-positive rate of 18%. In sub-analysis, in patients with one or more prior transfusion a sensitivity or 97% and a false-positive rate of 14% obtained at a threshold of greater than or equal to1.45 MoM. In patients with no previous transfusion the sensitivity for severe anemia was 100%, with a false-positive rate of 9.1% at a threshold value of greater than or equal to1.35 MoM. Conclusion: The middle cerebral artery peak systolic velocity can be used to time both the initial diagnostic procedure and retransfusion. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.