Neonatal alloimmune thrombocytopenia (NAIT) is a rare but clinically important etiology of intracranial hemorrhage. There have been no reported cases of intracranial hemorrhage caused by anti-group A or anti-group B antibodies. A Japanese boy weighing 1550 g was born at 37 weeks. He suffered from refractory thrombocytopenia and developed severe intracranial hemorrhage on his second day. Despite repeated platelet, red-cell and fresh-frozen-plasma transfusions, he died at day 10 of life. Serological studies and genotyping of the patient and his parents were performed. There were no incompatible genotypes of platelet antigens between the patient and the mother. Serological studies revealed that the mother had extremely high-titer anti-group A immunoglobulin G(2) (4096-fold) that reacted strongly with the father's platelets. The reaction against the father's platelets disappeared when her serum was adsorbed with group A red blood cells. Maternal anti-group A antibody was associated with NAIT and severe bilateral intracranial hemorrhage.