A total of 45 infants, including 20 appropriate-size-for-gestational-age infants (AGAs), 19 large-size-for-gestational-age infants (LGAs) and 6 infants of diabetic mothers (IDMs), had determinations of their pulmonary excretion rate of carbon monoxide (Veco) in the first postnatal week as an index of bilirubin production. We calculated a ratio (Rw) of birth weight to ideal weight (50th percentile for gestational age) as a relative measure of infant size. We also measured maternal glycosylated hemoglobin (Hb A~c) in the postpartum period as a reflection of the time-integrated blood glucose level over the weeks preceding delivery. Mean values for maternal Hb Ate in the postpartum period, infant Rw, and Veco were aI1 significantly increased for the LGAs and IDMs compared to the normal AGAs. Nine LGAs had mothers whose Hb A~o levels were >2 S.D. higher than the mean Hb A~ level for mothers of normal AGAs. The infants whose mothers had the highest Hb A~ levels were not always the ones with the highest bilirubin production rates. These findings suggest that maternal Hb Arc in the postpartum period, infant size, and bilirubin production are associated phenomena, but that a postpartum time-integrated measure of blood glucose level over the weeks preceding parturition may not reflect changes in other associated factors