Currently, the diagnosis of white matter necrosis may be performed with echoencephalography when cysts are observed in the white matter adjacent to the lateral ventricles. One hundred twenty-seven infants with a gestational age less than 36 weeks (mean [+/- SE] gestational age = 31 +/- 3.2 weeks) were studied in the neonatal period with echoencephalography to determine the incidence of white matter necrosis and the perinatal variables associated with this complication. Twenty-three infants (18.3%) had white matter necrosis. Thirteen (10.3%) had cysts by day 3 (11 on day 1), indicating that the onset of white matter necrosis occurred antenatally. The incidence of antenatal white matter necrosis was inversely related to birth weight and was more frequent in infants weighing less than 1000 gm (19%). Stepwise logistic regression analysis of 31 antenatal variables showed that placental vascular anastomoses in multiple pregnancies, funisitis, and purulent amniotic fluid were the only complications associated with antenatal white matter necrosis. Follow-up neurologic evaluations were abnormal in four of six patients with antenatal white matter necrosis. The findings in this study focus attention on prenatal, rather than intrapartum and postnatal, factors as causative agents of neurologic morbidity and emphasize the importance of early and sequential evaluation of neonatal brain structures.