The Di(b) antigen usually occurs with high incidence, except in certain Asian and South American Indian populations. In general, hemolysis caused by anti-Di(b) is not severe and its clinical course is benign. We report a Korean neonate with severe hemolytic disease of the newborn caused by anti-Di(b). The phenotype and genotype of the Diego blood group system of the patient and his mother were Di(a+b+) and Di(a+b-), respectively. The mother's serum and eluate from the neonate's erythrocytes contained anti-Di(b). This case was successfully managed with phototherapy and high dose iv immunoglobulin. Since most commercial antibody detection panels do not contain Di(b-) red cells, it is important to consider anti-Di(b) in cases of hemolytic disease of the newborn caused by an antibody against a high frequency antigen.