In a large Swedish 2 1/2-year population of fullterm infants the incidence of early neonatal sudden death (ENSD) was 0.12 per 1000 newborn infants considered healthy at birth but presenting with sudden cardiovascular collapse between 6 and 100 hours after birth. In the total neonatal mortality the rate of ENSD was 11%. Well-known risk infants, e.g. prematurity, perinatal asphyxia, difficult delivery or congenital malformations were not included. In an inborn population of 20 123 fullterm newborns the rate of ENSD was 0.15 per 1000. In the same population near-missed ENSD occurred in 0.35 per 1000 liveborn infants. Possible causative factors were infections in 4 and anaemia in 2 cases. But in 7 of 13 infants thorough epidemiological examinations and postmortem sterile autopsies did not reveal the cause of death or near death. These are probably cases of sudden infant death syndrome hitherto unrecognized in the first days of life.