A case of unilateral porencephalic cyst as a result of intravascular intrauterine transfusion is reported. Although other factors may have contributed, fetal bradycardia and relative hyperviscosity as a result of increased fetal hematocrit value were considered to be related to this event. Because some authors recommend direct intravascular transfusion to supraphysiologic hematocrits to prolong the interval between procedures, we would caution the use of this approach on the basis of theoretic considerations and the occurrence of this complication.