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 < 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 ≥ 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 ≥ 7.00 had such complications. Two of 23 infants with an umbilical artery pH < 7.00 had sequelae related to intrapartum asphyxia. In these two infants the umbilical artery pH was < 7.00, the 1-minute and 5-minute Apgar scores were ≤ 3 and the acidemia was metabolic in nature. © 1991, Mosby. All rights reserved.