Intrauterine intravascular transfusion is now believed to be a more precise method for treating fetal anemia in erythroblastosis fetalis than is intraperitoneal transfusion. Previously established guidelines for the volume of blood to be given in intraperitoneal transfusion at a specific gestational age are not applicable for intravascular transfusion. In 28 patients, intravascular transfusion was performed on 81 occasions between 19-34 weeks' gestation. The total number of transfusions ranged from one to six per patient. The aim at each procedure was to achieve a final hematocrit of 35-50%. Factors examined as likely to determine the volume of blood required included pre-transfusion hematocrit, post-minus pre-transfusion hematocrit (hematocrit increase), the hematocrit of the transfused blood, gestational age, estimated fetal weight, and interval from last transfusion. The factors found to be most predictive of total volume of blood required for transfusion were the hematocrit increase and either estimated fetal weight or gestational age.