The present multicenter study analysed the relative impact of maternal and infant factors on serum bilirubin levels at 72 ± 12 h in exclusively breastfed vs formula‐fed term infants. End‐tidal carbon monoxide levels corrected for ambient air (ETCOc), an index of bilirubin production, were measured in exclusively breastfed (B = 66) or formula‐fed (F = 210) term infants at 2–8 h of age. Inclusion criteria included cesarean section to ensure a 3 d hospitalization, birthweight ±2500 g, gestational age ±37 wk and absence of any illness. The ETCOc for B infants and F infants did not differ significantly (1.3 ± 0.7 ppm vs 1.3 ± 0.8 ppm). The serum bilirubin level at 72 ± 12 h was significantly higher in B infants than in F infants (8.5 ± 3.4mg dl−1 vs 6.7 ± 3.4 mg d l−1, p < 0.001), as was the percentage weight loss from birthweight. Serum bilirubin levels were significantly higher in infants who were male, who did not have meconium‐stained amniotic fluid, and in those whose mothers were insulin‐dependent diabetics or hypertensive. There was no difference between groups in the need for phototherapy or exchange transfusion. Conclusion: Although higher bilirubin levels were observed in group B at 72 ± 12 h compared with group F, this finding was not of clinical or therapeutic consequence in this study. The lack of difference in ETCOc between the groups may be a factor of the timing of ETCOc measurement in this study, or may suggest that early increased bilirubin production is not a significant contributor to jaundice observed in exclusively breastfed infants.