The degree of umbilical arterial acidemia associated with immediate newborn morbidity has not been determined. Therefore we compared 358 term infants with umbilical artery acidemia (pH less than 7.20) with 358 term, nonacidotic matched control infants, to evaluate immediate neonatal complications in both groups. Nonacidotic was defined as an umbilical artery pH greater than or equal to 7.20. Complications included seizures, persistent hypotonia, and/or signs of end-organ damage such as renal or cardiac dysfunction. None of the 693 newborns with an umbilical artery pH greater than or equal to 7.00 had such complications. Two of 23 infants with an umbilical artery pH less than 7.00 had sequelae related to intrapartum asphyxia. In these two infants the umbilical artery pH was less than 7.00, the 1-minute and 5-minute Apgar scores were less than or equal to 3 and the acidemia was metabolic in nature.