To evaluate the effect of the type of delivery, gestational age, maternal dexamethasone treatment, and neonatal complications on the serum cortisol levels in early infancy, a total of 92 neonates were investigated with 611 cortisol determinations (specific radioimmunoassay after Lipidex chromatography). Umbilical cord blood samples were taken immediately after delivery and capillary blood samples from the infant's heel 30-60 minutes after delivery and at 8:00 AM and PM on the second, fourth, and sixth days of life. Umbilical cord cortisol concentration after elective cesarean section was lower than after emergency cesarean section or after normal vaginal delivery, while neonatal cortisol values did not show any correlation with the type of delivery. Prematurity did not affect neonatal cortisol levels. In postterm infants the activation of cortisol production was retarded to some degree. After maternal dexamethasone therapy, neonatal cortisol concentration decreased 30-60 minutes after delivery, but from the second day on it was at the same level as in infants without maternal therapy. Respiratory distress syndome, especially in fatal cases, caused an elevation in neonatal cortisol levels, while hyperbilirubinemia did not have an effect on plasma cortisol concentrations of the neonates.