OBJECTIVE: The purpose of this study was to assess, in infants born weighing ≤1000 gm, if umbilical cord blood acid-base measures at birth are associated with an additional increase in neurosensory impairment. STUDY DESIGN: Of 289 surviving infants with a birth weight of 500 to 1000 gm born from 1979 to 1989, 219 had umbilical cord acid-base status measured at birth and were followed prospectively for ≥1 year. Measures of neurologic impairment used in this study included mental retardation, cerebral palsy, and major neurosensory impairment. RESULTS: Gestational age was inversely associated with all neurosensory impairments and was a better predictor of subsequent impairment in this population than was birth weight. Very low umbilical cord pH values were also significantly related to adverse outcomes. There was also an inverse relationship between cord blood bicarbonate levels and major neurosensory impairment. The highly significant relationship between cord blood bicarbonate and pH values and the development of neurosensory impairments persisted in spite of adjustment for gestational age, birth weight, plurality, use of general anesthesia, maternal race, and presence of hypertension. CONCLUSION: An adverse acid-base status at birth is additive to the effect of gestational age in predicting neurosensory impairment in infants weighing ≤1000 gm. (AM J OBSTET GYNECOL 1994;170:48-53.) © 1994, Mosby, Inc.. All rights reserved.