Cord blood thyroxine (T4) concentrations were measured in 4,068 infants from 28 wk gestation to term. Each chart was reviewed for the following factors: delivery by cesarean section, prolonged rupture of membranes, neonatal asphyxia, meconium-stained amniotic fluid, maternal diabetes mellitus and twinning. Each neonate was evaluated for the Idiopathic Respiratory Distress Syndrome, and low (SGA) or high (LGA) birthweight for gestational age. Within each gestational age group, the mean cord T4 value was similar except for a significantly lower mean cord T4 concentration for the term SGA subgroup. Thus, inclusion of the infant with a complicated neonatal course or the infant born to a high-risk mother in mass screening programs for congenital hypothyroidism using cord serum will not increase the number of false-positive T4 values.