One hundred twenty-seven infants less than 36 weeks of gestation (mean +/- SE = 31 +/- 3.2 weeks) were studied with echoencephalography to determine the incidence and complications associated with white matter necrosis. Ten infants (8%) developed cysts ten or more days after birth, indicating postnatal onset of white matter necrosis. Univariate analysis showed that postnatal white matter necrosis was significantly associated with maternal infection (other than urinary infection), respiratory distress syndrome, and longer requirement of an oxygen concentration greater than 40%. Forward logistic regression analysis showed postnatal white matter necrosis to be associated with maternal infection, chronic placental infarction, congenital pneumonia, and longer requirement of an oxygen concentration greater than 40%. Neurodevelopmental outcome was abnormal during infancy in 4 of the 6 survivors with postnatal white matter necrosis. Severe respiratory disease and maternal and/or fetal infection appear to increase the risk of the immature brain to white matter necrosis, predisposing the infants to subsequent neurodevelopmental delay.