We performed 41 intravascular ultrasound-guided fetal transfusions in a total of 20 pregnancies with erythroblastosis fetalis or alloimmune thrombocytopenia. On the basis of this experience, we developed a computer-assisted procedure for determining the volume to be transfused, which provides an adequate final concentration. Fetal weight was estimated using ultrasound measurements. Fetoplacental blood volume was estimated from the regression line: fetoplacental volume (ml) = 1.046 + fetal weight (g) X 0.14. The volume to be transfused was calculated using the simple dilution formula: Vtransfused = Vfetoplacental.(Cfinal - Cinitial)/Ctransfused where C is the hematocrit or platelet count. The entire procedure is computerized, simple and rapid, and avoids resorting to repeated intermediate sampling. The dilution formula used appears to be more reliable than a formula taking into account the volume added, even in erythroblastosis fetalis where relatively large volumes are injected. This suggests rapid plasma loss during the procedure.