Two hundred thirty term infants with measured acid-base status in umbilical arterial blood at birth were selected from 1210 consecutive deliveries for detailed neurodevelopmental follow-up at age 4 1/2 years; 203 were examined. Cutoff points approximately 1 SD from the mean (pH less than or equal to 7.10; base deficit greater than 12 mmol/L) were used to define acidosis. No statistically significant associations between acidosis and developmental outcome were found. The highest proportion of unimpaired children was found among those who were most severely acidotic at birth (pH less than or equal to 7.04; 2 SD below mean), but this finding was not statistically significant. These findings suggest that the ability of the fetus to produce an acidosis in response to the stress of labor may be beneficial to long-term outcome. The 10 nonacidotic babies with 1-minute Apgar scores of less than or equal to 3 showed statistically significant deficits in some areas. Coincident acidosis was not associated with a worse outcome for infants with low Apgar scores.