The purposes of this study were to evaluate the reliability of the previously described diagnostic criteria for Southeast Asian ovalocytosis (SAO) in adults in the diagnosis of SAO in newborns and to describe the role of SAO in newborn infants presenting with pallor and jaundice. The inclusion criteria in this retrospective descriptive study were that the patient be a newborn with pallor or jaundice and with ovalocytes in the peripheral blood smear (PBS). The exclusion criteria were newborn status with other causes of neonatal hemolysis or anemia. Controls were age-matched newborn infants who did not have SAO or other causes of neonatal anemia or hemolysis. Hematological data were assessed with a hematology analyzer. DNA analysis for SAO band 3 was done by polymerase chain reaction. Among 107 newborn infants with SAO, 30 infants were excluded from the study. The exclusions were premature infants, an infant with congenital syphilis, low-birth-weight infants, infants with ABO blood group incompatibility, infants with 3-thalassemia, infants with hemoglobin E heterozygote or homozygotes, glucose-6-phosphate dehydrogenase-deficient infants, and infants with fetomaternal hemorrhage. The DNA analysis for SAO band 3 was done in 56 newborns, and 54 had positive results for SAO band 3 gene deletion. Approximately one half of the 54 newborn infants with SAO had hyperbilirubinemia, and 3 had severe hyperbilirubinemia. The mean hemoglobin concentration, packed cell volume, and red blood cell (RBC) count in the infants with SAO in the first week of life were significantly lower than those in control infants. The mean absolute number of reticulocytes, mean corpuscular hemoglobin, and red cell volume distribution width in infants with SAO band 3 in the first week of life were significantly higher than those in control infants. The neonatal diagnosis of SAO can be made by examination of RBC morphology in the PBS with the presence of stomatocytes, theta cells, and > or = 25% ovalocytes. SAO plays a role in anemia and hyperbilirubinemia in newborn infants.