Objective: To investigate the hypothesis that meconium aspiration syndrome, the major hazard of meconium during labor, may be associated with superimposed fetal acute acidemia. Methods: Umbilical artery blood gases were measured in 7816 term pregnancies with meconium in the amniotic fluid (AF) and the results were correlated with intrapartum and neonatal outcomes. Results: Sixty-nine (1%) infants developed meconium aspiration syndrome and 31 (45%) of these were in association with fetal acidemia at birth. Moreover, umbilical blood gas analysis and intrapartum events suggested that the fetal acidemia linked to meconium aspiration was an acute event rather than a long-duration process, which might be expected if meconium was itself a marker of an antecedent fetal asphyxial event. Conclusion: Meconium in the AF may be a fetal environmental hazard when acidemia supervenes rather than solely a marker of preexisting fetal compromise leading to the release of meconium.