The development of 30 full-term infants undergoing surgery soon after birth was compared with a matched group of 29 healthy newborn infants in a prospective longitudinal study. At 1 year of age the infants who underwent surgery were performing within the normal range, but significantly less well than the controls in almost all areas of development. Of the various neonatal and perinatal factors studied, the length of hospital admission was the one most strongly associated with developmental progress at 1 year of age.