The purposes of our investigation were to establish normal umbilical cord blood acid-base values for breech-presenting fetuses born vaginally and to compare our findings with values from cephalic-presenting fetuses delivered vaginally (control group). Our study population consisted of 30 term fetuses with frank breech presentation who fulfilled selective criteria for trial of labor and subsequently had uncomplicated vaginal breech deliveries. The mean (+/- SD) gestational age of the study group was 38 weeks, 5 days (+/- 1 week, 4 days); the mean birth weight was 3178 +/- 321 g and the median 1- and 5-minute Apgar scores were 7 (range 2-9) and 8 (range 7-9), respectively. Thirteen infants had assisted breech deliveries and 17 had assisted breech deliveries with forceps applied to the aftercoming head. We obtained umbilical arterial and venous blood samples for acid-base analysis from both the study and control infants. The results for the study group were: umbilical venous pH 7.32, PCO2 37.88 mmHg, PO2 22.88 mmHg, HCO3 25.92 mmol/L, and base deficit -4.90; umbilical arterial pH 7.23, PCO2 51.20 mmHg, PO2 18.85 mmHg, HCO3 19.42 mmol/L, and base deficit -5.86. Comparison of the subjects' umbilical cord blood acid-base values with those of controls demonstrated a statistically lower cord arterial pH (P = .0001) and a higher arterial PCO2 (P = .028). We conclude that umbilical cord arterial blood acid-base values of the breech-presenting fetus born vaginally without complications differ from established normative umbilical cord blood gas values based on data obtained from uncomplicated cephalic-presenting fetuses delivered vaginally.