OBJECTIVE To study the distributions of pH and gas values in umbilical arterial (UA) blood of normal newborns following uncomplicated pregnancies and vaginal births. METHODS In 681 consecutive normal term infants who were born during 1990, we examined the UA pH and blood gas values obtained immediately following delivery. Maternal inclusion criteria were defined as an uncomplicated pregnancy and a normal spontaneous vaginal delivery. Umbilical artery blood samples were collected at each birth and were evaluated for pH, carbon dioxide pressure (PaCO2), oxygen pressure (PaO2), bicarbonate, and base excess. All newborns in this study had the following inclusion criteria: singletons, no malformations, growth appropriate for gestational age (AGA), and Apgar scores of 7 or more at 1, 5 and 10 min of life. RESULTS The lowest UA pH was 7.04 and the 10th percentile value was 7.21. The lowest UA PaO2 was 4.6 mmHg and the 10th percentile value was 10.1 mmHg. The highest UA PaCO2 was 75.4 mmHg and the 90th percentile 62 mmHg. The 10th percentile of the base deficit in the extracellular fluid was 5.9 mmol/l. CONCLUSIONS The distributions of the UA pH and gas values of a collective of normal newborns were illustrated.