Doppler ultrasound gives a measure of downstream resistance to blood flow. In this study it has been used to assess the change in placental vascular resistance following blood transfusion in pregnancies complicated by sickle cell disease. Eight women with homozygous sickle cell disease were followed prospectively to assess the effect of transfusion on haemoglobin S percentage, blood and plasma viscosity and uteroplacental Doppler velocimetry. Measurements were made immediately pre and post-transfusion and after several weeks when the desired improvement in haemoglobin S had been achieved. Despite significant changes in haemoglobin S levels and consequent rheological change there was found to be no change in measurements of resistance in the uteroplacental circulation as assessed by Doppler ultrasound.