Hypoxia has been associated with decreased gastric acidity in infants and children. In neonates, an inverse relationship between gestational age and gastric acidity may confound this association. To assess the relationship between oxygen supplementation and gastric acidity in premature neonates, mean 24-hour continuous measurements of gastric pH, obtained by an indwelling flexible gastric pH probe, were compared in infants requiring and not requiring oxygen. Patients were < 36 weeks' gestational age and were unfed before and during the assessment period. The gestational age of infants who required oxygen was less than that of infants who did not (30.5 +/- 3.0 vs 32.9 +/- 1.5 weeks, respectively). Other comparison variables were not different. The mean pH for infants requiring oxygen (4.4 +/- 1.7) was significantly greater than that of those not needing oxygen (2.7 +/- 1.2). After controlling for population characteristics and gestational age, we found that infants requiring oxygen still had significantly reduced gastric acidity.