Central nervous system hemorrhage is a well-recognized complication of neonatal alloimmune thrombocytopenia attributed to perinatal trauma from passage through the birth canal. That central nervous system (CNS) hemorrhage can occur in utero is not as well recognized, and congenital CNS lesions have only circumstantially been linked to thrombocytopenia. We report two cases of intrauterine CNS hemorrhage shown to have occurred prenatally, resulting in porencephaly. The second case is unique in that the necropsy finding of a porencephalic cyst arising from an old hemorrhagic site pathologically confirms that the etiology of congenital CNS lesions in alloimmune thrombocytopenia is due to hemorrhage. This second case received close prenatal monitoring and yet died as a result of hemorrhage that was not detected until emergency operative delivery. A review of the literature revealed 10 cases, including two other pairs of siblings, who had CNS damage attributable to intrauterine hemorrhage. These findings indicate that congenital CNS lesions in alloimmune thrombocytopenia are due to intrauterine hemorrhage that careful obstetric and prenatal care may not identify or prevent. This fact should be included when genetic counseling is offered to alloimmunized mothers.