OBJECTIVE Our purpose was to determine the effect of air contamination and the length of time before assay on umbilical cord blood gases at delivery. METHODS Umbilical venous cord blood gases (0.5 ml) were drawn from 21 patients at delivery in 1 and 3 ml heparinized syringes with varying amounts of air contamination (none, 0.5 ml air admitted and removed from the syringe, and 0.5 ml of air retained). In an additional 15 patients the 0.5 ml sample was contaminated with 0.1, 0.2, 0.3, 0.4, and 0.5 ml of air. RESULTS Air contamination had no effect on pH, PCO2, or bicarbonate but did increase the PO2 when the residual air was > 37.5% in a 3 ml syringe. No changes occurred in a 1 ml syringe. Time from drawing the blood to assay did not significantly alter the results. CONCLUSIONS Umbilical cord blood gases are best drawn in 1 ml heparinized syringes. Only if the residual air contamination exceeds 37.5% will PO2 increase. The level of air contamination will not alter the evaluation of fetal-neonatal acidosis.