Iy alloantigen system is the first polymorphism of platelet glycoprotein Ib beta reported to cause neonatal alloimmune thrombocytopenic purpura. We investigated the allelic frequency of Iy alloantigen among Japanese and Korean populations by polymerase chain reaction-restriction fragment length method to determine the possibility of alloimmunization against Iy. Two hundred and nine Japanese and 97 Korean subjects were examined. All 306 individuals were homozygous for glycine at amino acid position 15 and negative for Iy. The allelic frequency of Iy in these populations was calculated to be less than 0.0016. Alloimmunization associated with Iy antigen in Asian populations seems unlikely from these results.