We report a case of a sibling pair with neonatal alloimmune thrombocytopenic purpura. Serial antepartum platelet alloantibody quantitation by an enzyme-linked immunoabsorbent assay revealed rising antibody titers during advancing gestation. We discuss the implications of this finding in the antepartum diagnosis of neonatal alloimmune thrombocytopenic purpura, a rare, but frequently fatal disorder.