The incidence of hypoglycemia was determined in newborn infants in a general obstetric service. A random sample of patients was studied from nine birth weight-gestational age groups before the first feeding at 3 to 6 hours after birth. The highest incidence of hypoglycemia, 67% (serum glucose level < 30 mg/100 ml), occurred in the preterm SGA group. It was 25% in the term SGA infants and 18% in post-term SGA babies. Full term appropriately grown infants were noted to have a 10% incidence, and the preterm AGA group had a general shift toward lower prefeeding glucose levels. Infants of diabetic mothers were generally large for gestational age and delivered before term; hence, there was a 38% incidence of hypoglycemia in the preterm LGA group. When a serum glucose level of < 20 mg/100 ml was used to define hypoglycemia, the incidence in the preterm SGA groups was least reduced, i.e., to 40 and 21% respectively. Except for IDM's, evidence of intra-uterine growth retardation from the physical examination and confirmed by the weight/length ratio was demonstrated in infants who became hypoglycemic. An added stress in the form of birth hypoxia was present in the majority of the infants who became hypoglycemic. The combination of reduced energy reserves in newborn infants with intra-uterine growth retardation, plus the increased utilization of carbohydrates during birth hypoxia, resulted in a high incidence of neonatal hypoglycemia in the first few hours after birth.