We report a brother and sister, both of whom have porencephaly, hydrocephalus, optic atrophy, severe mental retardation, and spastic quadriplegia. In the younger child, abnormal intracranial structure was demonstrated by sonography at 32 weeks' gestation and was suspected earlier. Both children had transient severe thrombocytopenia as newborns. The mother is healthy and has never had purpura or other bleeding symptoms. However, her serum was found to react strongly with platelets from the father and from both children. The antibody in the mother's serum is platelet-specific but does not appear to be directed against any of the known antigens associated with neonatal alloimmune thrombocytopenic purpura (NATP) in other families, including PlA1, PlE2, or Baka. Although the mother's serum reacts with platelets from all 47 unrelated normal donors tested and from both the mothers and the fathers of 17 other children with suspected NATP, it does not react with her own platelets or with platelets from a patient with Glanzmann's thrombasthenia. These observations suggest that the serum from this woman identifies a previously undescribed high-frequency platelet-specific alloantigen and that sensitization to this determinant caused severe immune thrombocytopenia in both her children. It is likely that this led to intracranial hemorrhage in utero in these cases.