The relationship between cord blood gases and infection outcome was determined in 53 consecutive patients with preterm premature rupture of the membranes who were delivered because of abnormal fetal biophysical assessment. Measures of infection outcome included the presence of clinical amnionitis, possible neonatal sepsis, and neonatal sepsis. Fetal acidosis at birth, as defined by cord arterial pH less than 7.20, was found in six fetuses; five of these developed neonatal sepsis and the other was born to a mother who had intrapartum clinical amnionitis. The mean cord blood pH (artery and vein) of fetuses with neonatal sepsis was significantly less than in fetuses with possible neonatal sepsis or no sepsis; however, two thirds (10 of 15) of the neonates with sepsis had normal acid-base status at birth. These data suggest that the fetal biophysical assessment becomes abnormal before the development of fetal acidosis. The possible mechanisms by which fetal infection diminishes fetal biophysical activities prior to the development of acidosis are discussed.