Mortality of 908 immature births (newborn infants under 2501 g; stillbirths with crown-heel-lengths 35 cm plus) were studied and compared with a random selection of newborn infants with birth weights of at least 2500 g. Total mortality was 27,75%; 8,7% being stillbirths and 21,2% postnatal deaths; perinatal mortality was 26,98%. In mature groups a perinatal mortality of 0,64% was recorded. 90% of immature infants died within the first 3 days. In 30% of these cases cause of death was unknown, 30% died from immaturity, approx. 16% from apnoe and approx. 6% from cerebral haemorrhage.