OBJECTIVE: This study was undertaken to measure infant outcomes when pH at birth was compared with neonatal pH determined within 2 hours of age. STUDY DESIGN: We retrospectively studied term infants born between January 1, 1988, and August 31, 1998, who had umbilical artery blood pH measured at birth and again from the radial artery or umbilical artery within 2 hours after brth. Statistical significance was determined with the χ2 test. Odds ratios and 95% confidence intervals were calculated by means of the Mantel-Haenszel method. RESULTS: Data from a total of 1691 infants were analyzed: 178 (11%) had acidemia at birth (pH of <7.20) that persisted through the first 2 hours after birth; 110 (6%) had development of acidemia after birth; and 594 (35%) were born with a cord pH of <7.20 that improved after delivery. The remaining 809 infants (48%) did not have acidemia either at birth or during the neonatal period, and these served as the reference group. Seizures during the first 24 hours after birth were more likely among those infants with persistent acidemia (odds ratio, 13.0; 95% confidence interval, 6.3-26.7). The odds ratio for seizures among infants in whom acidemia developed after birth was 5.7 (95% confidence interval, 2.2-14.5). Other than the reference group, the infants who were born with acidemia that was corrected by 2 hours after birth had the lowest risk of seizures (odds ratio, 2.5; 95% confidence interval, 1.2-5.3). Significant differences in neonatal outcomes persisted after correction for anomalies. CONCLUSION: The direction of pH change from birth to the immediate neonatal period was significantly related to morbidity and mortality among term infants who were ill at birth or became ill shortly thereafter.